Ostentation of spinster seen out in new guise (15)
I believe the answer is:
pretentiousness
'ostentation' is the definition.
('pretentiousness' can be a synonym of 'ostentation')
'spinster seen out in new guise' is the wordplay.
'in new guise' indicates anagramming the letters (the same letters in a new form).
'spinster'+'seen'+'out'='spinsterseenout'
'spinsterseenout' with letters rearranged gives 'PRETENTIOUSNESS'.
'of' acts as a link.
(Other definitions for pretentiousness that I've seen before include "Pomposity, self-importance" , "Pseudery" .)