Unrivalled forward, trim, inhibited by nothing (9)
I believe the answer is:
nonpareil
'unrivalled' is the definition.
('nonpareil' can be a synonym of 'unrivalled')
'forward trim inhibited by nothing' is the wordplay.
'forward' becomes 'on'.
'trim' becomes 'pare' (I've seen this before).
'inhibited by' means one lot of letters goes inside another (I've seen this in other clues).
'nothing' becomes 'nil' ('nil' can be a synonym of 'nothing').
'on'+'pare'='onpare'
'onpare' put within 'nil' is 'NONPAREIL'.
(Other definitions for nonpareil that I've seen before include "unmatched!" , "Without equal or rival, from France" , "without peers" , "Unique type" , "beyond compare" .)