Mail about framer - sadly, it's always cold (10)
I believe the answer is:
permafrost
'always cold' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'mail about framer sadly' is the wordplay.
'mail' becomes 'post' (synonyms).
'about' means one lot of letters goes inside another.
'sadly' is an anagram indicator.
'framer' with letters rearranged gives 'ermafr'.
'post' going around 'ermafr' is 'PERMAFROST'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for permafrost that I've seen before include "Hard layer" , "Ground never thawing" , "Ground frozen for a long time" , "feature of polar regions" .)