Nose must be cold on top of K2 (4)
I believe the answer is:
conk
'nose' is the definition.
(informal term for nose)
'cold on top of k2' is the wordplay.
'cold' becomes 'c' (eg on taps).
'on' becomes 'o' (I've seen this in other clues).
'top of' becomes 'n' (I am not sure about this - if you are sure you should believe this answer much more).
'k2' becomes 'k'.
'c'+'o'+'n'+'k'='CONK'
'must be' acts as a link.
(Other definitions for conk that I've seen before include "(Slang) nose - head" , "Blow" , "Nose, head (sl.)" , "Slangy nose or head" , "Nose; break down" .)