Check inside friend's suits (7)
I believe the answer is:
matches
'suits' is the definition.
'check inside friend's' is the wordplay.
'check' becomes 'ch' (I've seen this in other clues).
'inside' means one lot of letters goes inside another.
'friend's' becomes 'mates' (mate is a kind of friend).
'ch' placed into 'mates' is 'MATCHES'.
(Other definitions for matches that I've seen before include "strikers found in the box?" , "ties" , "Contests or partnerships of equals" , "Contests or thin pieces of wood" , "Shares a colour with" .)