Stag groups always involve some violence (5)
I believe the answer is:
aggro
'violence' is the definition.
(I have seen 'Potential violence' mean 'aggro' so perhaps 'violence' could also mean 'aggro')
'stag groups always involve' is the wordplay.
'involve' indicates the answer is hidden within the clue (I've seen 'involves' mean this).
'AGGRO' is hidden within 'stag groups always'.
'some' acts as a link.
(Other definitions for aggro that I've seen before include "Fighting" , "Aggressive, violent behaviour in short" , "Belligerent trouble" , "Difficulties (informal)" , "Big trouble" .)