Tearing bits off flag, grossly offensive behaviour (5)
I believe the answer is:
aggro
'offensive behaviour' is the definition.
(I've seen this before)
'tearing bits off flag grossly' is the wordplay.
'tearing bits off' indicates a hidden word.
'AGGRO' is hidden within 'flag grossly'.
(Other definitions for aggro that I've seen before include "Nasty behaviour" , "Greek consumed by past" , "Slang for deliberate trouble-making" , "Boorish, threatening behaviour (slang)" , "Big trouble" .)