Tense opponent must accept one is insignificant (7)
I believe the answer is:
trivial
'insignificant' is the definition.
(similar in meaning)
'tense opponent must accept one' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'opponent' becomes 'rival' (I've seen this before).
'must accept' means one lot of letters goes inside another.
'one' becomes 'i' (roman numeral).
'rival' going around 'i' is 'rivial'.
't'+'rivial'='TRIVIAL'
'is' acts as a link.
(Other definitions for trivial that I've seen before include "Quite unimportant and paltry" , "Meaningless" , "Petty, piffling" , "Insignificant" , "Rinky-dink" .)