Stag party undressed by first of hens (4)
I believe the answer is:
hart
'stag' is the definition.
(both can mean a male deer)
'party undressed by first of hens' is the wordplay.
'undressed' says to take the centre (the word removes the letters which clothe it).
'by' says to put letters next to each other.
'first of hens' becomes 'h' (1st letter of 'hens').
The centre of 'party' is 'art'.
'art' after 'h' is 'HART'.
(Other definitions for hart that I've seen before include "Richard Rodgers and Lorenz . . . . were famous musical partnership" , "Beast" , "Songwriting partnership, Rodgers and --" , "Mature male deer" , "Animal" .)