The full fifty enter to take part in (8)
I believe the answer is:
complete
'in' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'the full fifty enter to take part' is the wordplay.
'the full fifty' becomes 'l' (Roman numeral. I am not sure about the 'full' bit.).
'enter' means one lot of letters goes inside another.
'to take part' becomes 'compete'.
'l' inserted into 'compete' is 'COMPLETE'.
Can you help me to learn more?
(Other definitions for complete that I've seen before include "accomplished" , "Whole, entire, uncut" , "Intact" , "Whole and unabridged" , "Done" .)