Something on a mountain, say, that is restricting cold? A hat (6)
I believe the answer is:
icecap
'something on a mountain say' is the definition.
(a mountain might be covered by an icecap)
'that is restricting cold? a hat' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'restricting' indicates putting letters inside.
'cold?' becomes 'c' (eg on taps).
'a hat' becomes 'cap' (I've seen this before).
'ie' placed around 'c' is 'ice'.
'ice'+'cap'='ICECAP'
(Other definitions for icecap that I've seen before include "Cold mass permanently covering the polar regions or mountain peaks" , "Frozen polar region" , "Permanent covering of frozen water" , "mass covering the top of Everest?" , "Permanent coating of white on a mountain-top" .)