The whole fifty will enter to contend (8)
I believe the answer is:
complete
'the whole' is the definition.
(I've seen this before)
'fifty will enter to contend' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'will enter' indicates putting letters inside.
'to contend' becomes 'compete' ('compete' can be a synonym of 'contend').
'l' put into 'compete' is 'COMPLETE'.
(Other definitions for complete that I've seen before include "Finished - whole" , "Thorough" , "Finish; whole" , "Intact" , "Whole, entire, uncut" .)