Posh leading man is perfect (7)
I believe the answer is:
utopian
'is 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 "Aspiring to impracticable perfection" , "Perfect (state)" , "Of supposed perfection" , "Too perfect for this world" , "Unworkable" .)