Plain little girl about to enter sporting contest (7)
I believe the answer is:
evident
'plain' is the definition.
('evident' can be a synonym of 'plain')
'little girl about to enter sporting contest' is the wordplay.
'little girl' becomes 'di' (short for Diane, Diana etc).
'about' is a reversal indicator.
'to enter' is an insertion indicator.
'sporting contest' becomes 'event' (I've seen this before).
'di' reversed gives 'id'.
'id' put into 'event' is 'EVIDENT'.
(Other definitions for evident that I've seen before include "clearly visible" , "Comprehensible - manifest" , "Clearly apparent" , "Plain, obvious, clear" , "Visible - plain" .)