One accepting fine is unsuitable (5)
I believe the answer is:
unfit
'unsuitable' is the definition.
('unfit' can be similar in meaning to 'unsuitable')
'one accepting fine' is the wordplay.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'accepting' is an insertion indicator.
'fine' becomes 'f' (abbreviation).
'unit' going around 'f' is 'UNFIT'.
'is' is the link.
(Other definitions for unfit that I've seen before include "struggling to keep up?" , "In poor condition" , "In bad physical condition" , "not worthy" , "Inapt" .)