It’s my fault henhouses won’t open (4)
I believe the answer is:
oops
'it's my fault' is the definition.
'henhouses won't open' is the wordplay.
'henhouses' becomes 'coops' ('coop' can be a synonym of 'henhouse').
'won't open' means to remove the first letter.
'coops' with its first letter taken away is 'OOPS'.
(Other definitions for oops that I've seen before include "Oh dear, that's a mistake!" , "Exclamation recognising a mistake has been made" , "Apologetic exclamation" , "Exclamation on making obvious mistake" , "I've made a mistake!" .)