Poet’s mostly gentle expression? Mostly (6)
I believe the answer is:
milton
'poet's' is the definition.
(John Milton)
'mostly gentle expression? mostly' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'gentle' becomes 'mild' (similar in meaning).
'expression? mostly' becomes 'ton' (I can't justify this - if you can you should believe this answer much more).
'mild' with its last letter removed is 'mil'.
'mil'+'ton'='MILTON'
Can you help me to learn more?
(Other definitions for milton that I've seen before include "Bucks new town" , "English epic poet" , "Blind poet of ` `Paradise Lost' '" , "Paradise Lost author" , "and 24: Town in Bucks" .)