Take part not all of whole (missing fifth) (7)
I believe the answer is:
compete
'take part' is the definition.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for compete that I've seen before include "Vie, contest" , "Take part (in race)" , "Strive (against others)" , "Engage in a contest" , "Struggle for victory" .)