Deserve one term? Doubtful! (5)
I believe the answer is:
merit
'deserve' is the definition.
(synonyms)
'one term? doubtful' is the wordplay.
'one' becomes 'i' (Roman numeral).
'doubtful' is an anagram indicator.
'i'+'term'='iterm'
'iterm' is an anagram of 'MERIT'.
(Other definitions for merit that I've seen before include "Excellence that deserves to be honoured" , "credit" , "Quality deserving reward" , "The M in OM" , "Advantage" .)