Taking part in protest against deer (4)
I believe the answer is:
stag
'deer' is the definition.
(stag is a kind of deer)
'taking part in protest against' is the wordplay.
'taking part in' says the answer is hidden in the clue.
'STAG' can be found hidden inside 'protest against'.
(Other definitions for stag that I've seen before include "Excluding women" , "American party" , "Deer; with only men" , "involving men" , "Male animal" .)