Nominal fine received by a number (5)
I believe the answer is:
token
'nominal' is the definition.
(synonyms)
'fine received by a number' is the wordplay.
'fine' becomes 'ok' (synonyms).
'received by' means one lot of letters goes inside another.
'a number' becomes 'ten' (ten is a kind of number).
'ok' inserted inside 'ten' is 'TOKEN'.
(Other definitions for token that I've seen before include "Nominal; voucher" , "Privately issued coin or voucher" , "Symbolic; voucher" , "Disc used as money or a gift card" , "Done for appearance's sake" .)