Henhouses with no roof? My mistake! (4)
I believe the answer is:
oops
'my mistake' is the definition.
(oops may be said to acknowledge a mistake)
'henhouses with no roof?' is the wordplay.
'henhouses' becomes 'coops' ('coop' can be a synonym of 'henhouse').
'with no roof?' means to remove the first letter (in a down clue, roof could mean the top letter).
'coops' with its initial letter taken away is 'OOPS'.
(Other definitions for oops that I've seen before include "I've messed up" , "Oh dear, I've spilt something!" , "Exclamation recognising a mistake has been made" , "The sound of stupidity" , "Something's been spilt!" .)