Competitor having to dine in suit (8)
I believe the answer is:
entreaty
'suit' is the definition.
(I know that suit can be written as entreaty)
'competitor having to dine' is the wordplay.
'competitor' becomes 'entry' (entry in a competition).
'having' means one lot of letters goes inside another.
'to dine' becomes 'eat' (eat a meal).
'entry' enclosing 'eat' is 'ENTREATY'.
'in' is the link.
(Other definitions for entreaty that I've seen before include "Teeny rat (anag.)" , "petition" , "Supplication" , "application for assistance" , "Earnest or urgent request" .)