Cockney crook cooked a fat eel (3,4)
I believe the answer is:
tea leaf
'cockney crook' is the definition.
(I've seen this before)
'cooked a fat eel' is the wordplay.
'cooked' indicates an anagram.
'a'+'fat'+'eel'='afateel'
'afateel' with letters rearranged gives 'TEA-LEAF'.
(Other definitions for tea leaf that I've seen before include "Darjeeling?" , "one of many going to pot?" , "Dried part of a plant, used to make cuppas" , "ending up in hot water" , "Cockney crook" .)