I see a key as something cold (3)
I believe the answer is:
ice
'something cold' is the definition.
'i see a key' is the wordplay.
'see' becomes 'c' (the word for the letter, according to Chambers).
'a key' becomes 'e' (I've seen this in other clues).
'i'+'c'+'e'='ICE'
'as' is the link.
(Other definitions for ice that I've seen before include "Diamonds (colloq.)" , "Form of water" , "Some number" , "Solid H2O" , "Finish decorating the Christmas cake" .)