Posh baronet in cultivated style shows refinement (8)
I believe the answer is:
subtlety
'refinement' is the definition.
(I've seen this before)
'posh baronet in cultivated style' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'baronet' becomes 'bt'.
'in' is an insertion indicator.
'cultivated' indicates anagramming the letters.
'u'+'bt'='ubt'
'style' anagrammed gives 'slety'.
'ubt' inserted within 'slety' is 'SUBTLETY'.
'shows' is the link.
(Other definitions for subtlety that I've seen before include "refinement" , "Finesse" , "nice" , "Delicate/refined quality" , "Ornamental device in sugar" .)