Finish challenge that's over long (8)
I believe the answer is:
complete
'finish' is the definition.
(I know that finish can be written as complete)
'challenge that's over long' is the wordplay.
'challenge' becomes 'compete' (to compete against someone is to challenge them).
'that's over' is an insertion indicator.
'long' becomes 'l'.
'compete' going around 'l' is 'COMPLETE'.
(Other definitions for complete that I've seen before include "Perfect - finish" , "Intact" , "Having all the necessary parts" , "Whole and unabridged" , "Full" .)