Posh leading man - perfect! (7)
I believe the answer is:
utopian
'perfect' is the definition.
(associated in meaning)
'posh leading man' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'leading' becomes 'top' (both can mean first-class).
'man' becomes 'ian' (man's name).
'u'+'top'+'ian'='UTOPIAN'
(Other definitions for utopian that I've seen before include "Fantastic" , "ideal but unrealistic" , "Of an ideal existence" , "Perfect (state)" , "Ideal; imaginary" .)