Chicken when headless (3)
I believe the answer is:
hen
'chicken' is the definition.
(hen is a kind of chicken)
'when headless' is the wordplay.
'headless' means to remove the first letter.
'when' with its first letter taken off is 'HEN'.
(Other definitions for hen that I've seen before include "Domestic fowl" , "This party is for women only" , "Domestic egg layer" , "Laying bird" , "Female lobster, fowl" .)