Old artist upset about Girl Guides’ violent behaviour (5)
I believe the answer is:
aggro
'violent behaviour' is the definition.
(aggression)
'old artist upset about girl guides'' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'upset' shows that the letters should be reversed in order.
'about' means one lot of letters goes inside another.
'girl guides'' becomes 'GG' (abbreviation).
'o'+'ra'='ora'
'ora' backwards is 'aro'.
'aro' placed around 'gg' is 'AGGRO'.
(Other definitions for aggro that I've seen before include "Hostile troublemaking" , "Aggressive, violent behaviour in short" , "Slang for deliberate trouble-making" , "Grief" , "offensive behaviour" .)