Inappropriate fine accepted by one (5)
I believe the answer is:
unfit
'inappropriate' is the definition.
(thesaurus)
'fine accepted by one' is the wordplay.
'fine' becomes 'f' (abbreviation).
'accepted by' is an insertion indicator.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'f' placed into 'unit' is 'UNFIT'.
(Other definitions for unfit that I've seen before include "Ill - out of shape" , "In poor condition" , "Incapable" , "Not prepared" , "Not in good physical condition, or not suitable" .)