Some prize stimulates enthusiasm (4)
I believe the answer is:
'enthusiasm' is the definition.
(zest can mean enthusiasm or eagerness)
'some prize stimulates' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'ZEST' is hidden within 'prize stimulates'.
(Other definitions for zest that I've seen before include "Outer layer of citrus peel" , "Enthusiasm - grated lemon peel" , "Piquancy - fruit rind" , "Piquancy (eg from lemon peel)" , "Vigour" .)