Hostility within stag group (5)
I believe the answer is:
aggro
'hostility' is the definition.
(short for aggression or aggravation)
'within stag group' is the wordplay.
'within' indicates a hidden word.
'AGGRO' is hidden within 'stag group'.
(Other definitions for aggro that I've seen before include "Nasty behaviour" , "Hostile troublemaking" , "Difficulties (informal)" , "Violence - obstructiveness" , "In slang, hassle or troublemaking" .)