Proscribed label on pair of spectacles (5)
I believe the answer is:
taboo
'proscribed' is the definition.
('taboo' can be a synonym of 'proscribed')
'label on pair of spectacles' is the wordplay.
'label' becomes 'tab' (I've seen this before).
'on' means one lot of letters go next to another.
'pair of spectacles' becomes 'oo' (OO looks like spectacles).
'tab'+'oo'='TABOO'
(Other definitions for taboo that I've seen before include "Not allowed, forbidden, proscribed" , "Prohibited as unclean or holy" , "Beyond the pale" , "Socially prohibited" , "Prohibited; ban" .)