Propose a short letter about main changes (8)
I believe the answer is:
nominate
'propose' is the definition.
('nominate' can be a synonym of 'propose')
'a short letter about main changes' is the wordplay.
'a short letter' becomes 'note' ('note' can be a synonym of 'short letter').
'about' is an insertion indicator.
'changes' is an anagram indicator.
'main' with letters rearranged gives 'mina'.
'note' going around 'mina' is 'NOMINATE'.
(Other definitions for nominate that I've seen before include "Choose" , "Propose - a mention (anag)" , "Specify (person for job)" , "Propose - name" , "Propose for office" .)