Add it to cheese and form a frozen layer (3,5)
I believe the answer is:
ice sheet
'form a frozen layer' is the definition.
The definition suggests a singular noun which matches the answer.
'add it to cheese' is the wordplay.
I cannot really see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is located in the answer.
an anagram of 'cheese' is 'ceshee' which is located in the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'and' acts as a link.
Can you help me to learn more?
(Other definitions for ice sheet that I've seen before include "Extensive frozen layer over land" , "Extensive frozen area" , "white surface layer" .)