Return of banknote, which may be in Leicester, say (6)
I believe the answer is:
rennet
'which may be in leicester say' is the definition.
(rennet is used to make cheese, eg Red Leicester)
'return of banknote' is the wordplay.
'return of' says the letters should be written in reverse.
'banknote' becomes 'tenner' (I've seen this before).
'tenner' written backwards gives 'RENNET'.
(Other definitions for rennet that I've seen before include "Milk curdling agent" , "It's used to curdle milk for cheese" , "Substance used to curdle milk for cheese-making" , "cheesy setter?" , "Agent for cheesemaker" .)