Fish in mostly beer with leafy green (8)
I believe the answer is:
pilchard
'fish' is the definition.
(pilchard is a kind of fish)
'mostly beer with leafy green' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'beer' becomes 'pils' (Pilsner beer).
'with' says to put letters next to each other.
'leafy green' becomes 'chard' (I've seen this in another clue).
'pils' with its last letter taken away is 'pil'.
'pil'+'chard'='PILCHARD'
'in' acts as a link.
(Other definitions for pilchard that I've seen before include "Edible herring relative" , "Small fatty fish, usually tinned" , "Small sea fish related to the herring" , "Fish of herring family" , "Small marine food fish" .)