Swimmer up mountain gets chill (6)
I believe the answer is:
plaice
'swimmer' is the definition.
(plaice is a kind of fish)
'up mountain gets chill' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'mountain' becomes 'alp' (the Alps are a mountain range).
'gets' is a charade indicator (letters next to each other).
'chill' becomes 'ice' (icing is a kind of chilling).
'alp' reversed gives 'pla'.
'pla'+'ice'='PLAICE'
(Other definitions for plaice that I've seen before include "Flatfish; sounds like put" , "Ice, pal, will catch this fish" , "A succulent flat-fish" , "A fish; sounds like put" , "Flatfish, valued as food" .)