White coating on a mountain-top (6)
I believe the answer is:
icecap
'white coating on a mountain-top' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for icecap that I've seen before include "mass covering the top of Everest?" , "is always frozen" , "covering in frozen north" , "Permanent covering in frozen Arctic regions" , "Permanent polar covering" .)