Hormone replacement for bird (7)
I believe the answer is:
moorhen
'for bird' is the definition.
(moorhen is a kind of bird)
'hormone replacement' is the wordplay.
'replacement' indicates an anagram.
'hormone' anagrammed gives 'MOORHEN'.
(Other definitions for moorhen that I've seen before include "Black waterbird" , "Water bird" , "Black freshwater bird" , "pond bird" , "Medium-sized, blackish aquatic bird with long toes" .)