Some scallywag group's belligerent behaviour (5)
I believe the answer is:
aggro
'belligerent behaviour' is the definition.
(I've seen this before)
'some scallywag group's' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'AGGRO' can be found hidden inside 'scallywag group'.
(Other definitions for aggro that I've seen before include "belligerent behaviour" , "Violence - obstructiveness" , "Hostility" , "Difficulties, trouble" , "In slang, hassle or troublemaking" .)