50 entering contest finished (8)
I believe the answer is:
complete
'finished' is the definition.
(I've seen this before)
'50 entering contest' is the wordplay.
'50' becomes 'l' (Roman numeral).
'entering' means one lot of letters goes inside another.
'contest' becomes 'compete' (I've seen this before).
'l' going within 'compete' is 'COMPLETE'.
(Other definitions for complete that I've seen before include "accomplished" , "Finished, entire" , "Execute" , "Thorough" , "Whole, nothing missing" .)