Two fivers returned for a cheese ingredient (6)
I believe the answer is:
rennet
'a cheese ingredient' is the definition.
(curdled milk used in cheese)
'two fivers returned' is the wordplay.
'two fivers' becomes 'tenner' (I've seen this in another clue).
'returned' says the letters should be written backwards.
'tenner' written backwards gives 'RENNET'.
'for' acts as a link.
(Other definitions for rennet that I've seen before include "Cheese-making curdler" , "Causing vomiting" , "Preparation for curdling milk" , "Substance curdling milk in cheese-making" , "Tenner (anag.); cheese ingredient" .)