Swimmer up mountain to freeze (6)
I believe the answer is:
plaice
'swimmer' is the definition.
(plaice is a kind of fish)
'up mountain to freeze' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'mountain' becomes 'alp' (the Alps are a mountain range).
'to freeze' becomes 'ice' (as in water freezing to become ice).
'alp' backwards is 'pla'.
'pla'+'ice'='PLAICE'
(Other definitions for plaice that I've seen before include "Flat fare" , "Edible flatfish" , "Tasty flatfish I place here" , "Large flat fish" , "Flatfish; sounds like put" .)