Legal proceeding supports fine for splinter group (7)
I believe the answer is:
faction
'splinter group' is the definition.
(as in a faction of an organisation)
'legal proceeding supports fine' is the wordplay.
'legal proceeding' becomes 'action' (action is a kind of legal proceeding).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'fine' becomes 'f' (abbreviation).
'action' put after 'f' is 'FACTION'.
'for' acts as a link.
(Other definitions for faction that I've seen before include "Minority who dissents from main group" , "band" , "camp" , "Rebellious group" , "Dissenting group within a larger one" .)