Idealistic reformer best protected by posh Scotsman (7)
I believe the answer is:
utopian
'idealistic reformer' is the definition.
(I've seen this before)
'best protected by posh scotsman' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'protected by' means one lot of letters goes inside another (inserted letters are held or protected).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'scotsman' becomes 'ian' (common Scottish name).
'u'+'ian'='uian'
'top' placed inside 'uian' is 'UTOPIAN'.
(Other definitions for utopian that I've seen before include "More work was literally so" , "Thomas More-ish?" , "Ideal; imaginary" , "Perfectionist reformer" , "Just perfect, as in Thomas More's ideal country" .)