One entering time that's competitive — not this? (7)
I believe the answer is:
trivial
'not this?' is the definition.
The definition suggests a singular noun but the answer is not.
'one entering time that's competitive' is the wordplay.
'one' becomes 'i' (Roman numeral).
'entering' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'that's competitive' becomes 'rival' (similar in meaning).
't'+'rival'='trival'
'i' placed within 'trival' is 'TRIVIAL'.
(Other definitions for trivial that I've seen before include "Of little value" , "Frivolous" , "Insignificant" , "lacking weight" , "Of little consequence" .)