A little mollusc: it crawls backwards when very cold (6)
I believe the answer is:
arctic
'very cold' is the definition.
(I've seen this before)
'a little mollusc it crawls backwards' is the wordplay.
'a little' says the answer is hidden in the clue.
'backwards' shows that the letters should be reversed in order.
'citcra' is hidden in the letters of 'mollusc it crawls'.
'citcra' reversed gives 'ARCTIC'.
'when' is the link.
(Other definitions for arctic that I've seen before include "Of regions around the North Pole" , "Northern ocean" , "Very cold (weather)" , "Far-northern zone" , "Relating to the cold North" .)