Language spoken by a Cockney’s wife? (5)
I believe the answer is:
dutch
This clue is a double definition.
'language spoken' is the first definition.
(language of the Netherlands)
'a cockney's wife?' is the second definition.
(Cockney informal term for a wife)
'by' acts as a link.
(Other definitions for dutch that I've seen before include "Sort of courage" , "Better half" , "- Auction; - barn" , "& 7 Alcohol-induced nerve" , "partner" .)