Get on to the subject of British fish (6)
I believe the answer is:
broach
'get on to the subject' is the definition.
I can't judge whether this defines the answer.
'british fish' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'fish' becomes 'roach' (roach is a kind of fish).
'b'+'roach'='BROACH'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for broach that I've seen before include "Pierce (a cask)" , "Start to speak" , "Bring up for discussion" , "Begin" , "Raise (a subject) for discussion" .)