Disobedient brat involved with union side (13)
I believe the answer is:
insubordinate
'disobedient' is the definition.
(similar in meaning)
'brat involved with union side' is the wordplay.
'involved with' is an anagram indicator (involved can mean confusing or complex).
'union'+'side'='unionside'
'brat' mixed up with 'unionside' gives 'INSUBORDINATE'.
(Other definitions for insubordinate that I've seen before include "Badly behaved" , "Recalcitrant" , "Rebellious" , "Disobedient" , "Insolent" .)