Mountain retreat needs freezer for fish (6)
I believe the answer is:
plaice
'for fish' is the definition.
(plaice is a kind of fish)
'mountain retreat needs freezer' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'retreat' says the letters should be written backwards.
'needs' is a charade indicator (letters next to each other).
'freezer' becomes 'ice' (something which 'freezes').
'alp' written backwards gives 'pla'.
'pla'+'ice'='PLAICE'
(Other definitions for plaice that I've seen before include "The fish sounds as if it hits the spot" , "Large flat fish" , "A fish; sounds like put" , "Large flatfish" , "Flatfish, valued as food" .)