Dairy product not British, say (5)
I believe the answer is:
'say' is the definition.
(to utter something is to speak or say it)
'dairy product not british' is the wordplay.
'dairy product' becomes 'butter' (butter is a kind of dairy product).
'not' is a deletion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'butter' with 'b' taken away is 'UTTER'.
(Other definitions for utter that I've seen before include "Speak, articulate" , "Express audibly" , "Sheer" , "Articulate, express" , "Total < state" .)