Plain, I'd entered sporting contest (7)
I believe the answer is:
evident
'plain' is the definition.
(I know that plain can be written as evident)
'i'd entered sporting contest' is the wordplay.
'entered' is an insertion indicator.
'sporting contest' becomes 'event' (I've seen this before).
'id' going into 'event' is 'EVIDENT'.
(Other definitions for evident that I've seen before include "Quite obvious, manifest" , "Patent" , "Obvious - 1 across" , "Clearly apparent" , "Conspicuous" .)