What comes from opening letters, thousand written by a mate (7)
I believe the answer is:
acronym
'what comes' is the definition.
I can't judge whether this definition defines the answer.
'opening letters thousand written by a mate' is the wordplay.
'opening' means one lot of letters goes inside another (some letters open a gap in others).
'letters' indicates anagramming the letters.
'thousand' becomes 'm' ('m' can be a synonym of 'thousand').
'written by' means one lot of letters go next to another.
'mate' becomes 'crony' (I've seen this before**).
'm'+'a'='ma'
'ma' anagrammed gives 'am'.
'am' going around 'crony' is 'ACRONYM'.
'from' is the link.
Can you help me to learn more?
(Other definitions for acronym that I've seen before include "NATO's one" , "Ernie, perhaps" , "UNICEF, perhaps" , "radar, for one" , "Tessa, say" .)