WW2 leader’s some competitor! (4)
I believe the answer is:
tito
'ww2 leader's' is the definition.
(Josip Tito)
'some competitor' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'TITO' is hidden within 'competitor'.
(Other definitions for tito that I've seen before include "Josip Broz pseudonym" , "President of Yugoslavia, 1953-80" , "Revolutionary" , "Statesman once" , "Yugoslav leader, died 1980" .)