Leo's cooked a fish (4)
I believe the answer is:
sole
'a fish' is the definition.
(sole is a kind of fish)
'leo's cooked' is the wordplay.
'cooked' indicates an anagram.
'leos' with letters rearranged gives 'SOLE'.
(Other definitions for sole that I've seen before include "European flatfish" , "A lonely sort of flatfish" , "Just one - fish" , "Underside of the foot" , "Dover - -" .)