Bring up book on freshwater fish (6)
I believe the answer is:
broach
'bring up' is the definition.
(I've seen this before)
'book on freshwater fish' is the wordplay.
'book' becomes 'b'.
'on' means one lot of letters go next to another.
'freshwater fish' becomes 'roach' (I've seen this before).
'b'+'roach'='BROACH'
(Other definitions for broach that I've seen before include "Raise (a sensitive topic)" , "crack" , "Bring up a topic" , "Bring up or mention a subject" , "Bring up for discussion" .)