Extract of a letter from Clay County, Mo. to the Editor of the Missouri Republican, July 29th.

'The bearer hereof has been employed by the commanding officer at Cantonment Leavenworth as an express to Jefferson Barracks, asking aid from that post to protect the public property left at Cantonment Leavenworth, by the 3d regiment and the detachment of the 6th, under Major Riley. On Monday morning, 27th, forty of our citizens volunteered for the defence of that post, and left this place for it about ten o'clock. Lieutenant Lee, the Commander at that post, called upon the Colonel of our county officially for this assistance. We do not know of the absolute necessity of our citizens to be called upon to enter into actual service; but there are so many concurring circumstances that render it highly probable that that post will be attacked, that this precautionary step is approved of by all.

'Some Delaware Indians, on their return from Major Riley's detachment on the Little Arkansas, have been attacked by the Pawnees-some killed on each side, and the scalp of at least one Pawnee has been produced to Lieutenant Lee, as evidence of the encounter. About two weeks ago, an express, consisting of two men, was sent from Cantonment Leavenworth to Maj. Riley, with official communications, 'c. These Delawares assert that those men bearing the express have been killed by the Pawnees. The express sent to Jefferson Barracks is to advise the commanding officers of that post of these facts, and to ask a part of the military force now lying idle at that place to be sent up for the defence of this section of the country.

The citizens of this county, as those in every other county in the Boon's Lick country, are in quite an uproar. On Monday, 250 men volunteered in this county, prepared to march at a moment's warning. The step has been taken in pursuance of the requisition of Brigadier General Thompson, of the militia. From Howard, we hear the alarm is much greater than with us--as it is said 7 or 800 warriors from the Miss. are in the vicinity of that place. From your situation, you will learn the particulars from that county as soon as we do here.

I should not omit to mention, that as soon as Major Dougherty heard of the renconter (sic) in Randolph between our citizens and the Indians, he dispatched Major Bean Sub-Indian Agent, to the village of the Ioways; to counsel with them. It was reported that it was those Indians engaged in Randolph- but those at the village disclaimed any knowledge of the affray, and expressed a readiness to cooperate to bring the aggressors to punishment, if it should turn out that their men were concerned in it. At the same time he was with the Ioways, 60 or 80 of the Mississippi Indians arrived at the Ioway and Sac villages--all warriors. Their object could not be ascertained by Major Bean, and therefore he looks upon the arrival at that particular moment as ominous.

We cannot but regret the evacuation of the post at Cantonment Leavenworth, as much of the unfriendly and hostile feelings of the Indians upon the Missouri arises from the unprotected and defenceless (sic) situation of that post.'