Wanting, during contest, to cross line (10)
I believe the answer is:
incomplete
'wanting' is the definition.
(I've seen this before)
'during contest to cross line' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'contest' becomes 'compete' (I've seen this before).
'to cross' is an insertion indicator (some letters cross or go round others).
'line' becomes 'l' (used when specifying particular lines from a poem).
'compete' enclosing 'l' is 'complete'.
'in'+'complete'='INCOMPLETE'
(Other definitions for incomplete that I've seen before include "Not whole" , "Inadequate" , "Unfinished" , "not sufficient" , "not done" .)