"Fish" is no good in beer! (5)
I believe the answer is:
angle
'fish' is the definition.
(angle can mean to fish)
'no good in beer' is the wordplay.
'no good' becomes 'ng' (abbreviation).
'in' means one lot of letters goes inside another.
'beer' becomes 'ale' (type of beer).
'ng' inserted into 'ale' is 'ANGLE'.
'is' acts as a link.
(Other definitions for angle that I've seen before include "Standpoint" , "Fish; viewpoint" , "Early settler" , "Point of view - associated with Saxon" , "Opinion" .)