Bird subsequently losing head (3)
I believe the answer is:
hen
'bird' is the definition.
(hen is a kind of bird)
'subsequently losing head' is the wordplay.
'subsequently' becomes 'then' ('then' can be a synonym of 'subsequently').
'losing head' means to remove the first letter.
'then' with its initial letter taken off is 'HEN'.
(Other definitions for hen that I've seen before include "Domestic fowl" , "Laying bird" , "Adult female chicken" , "Farm bird" , "Not cock" .)