Man involved with a girl on the side (8)
I believe the answer is:
marginal
'side' is the definition.
(thesaurus)
'man involved with a girl on' is the wordplay.
'involved with' is an anagram indicator (involved can mean confusing or complex).
'on' says to put letters next to each other.
'a'+'girl'='agirl'
'man' mixed up with 'agirl' can make 'MARGINAL'.
'the' is the link.
(Other definitions for marginal that I've seen before include "Of minor importance" , "Very slight in amount, at the edge" , "On the edges of" , "on border" , "Borderline" .)