Visual aid gets royal person (not king) backing doctrine (5-3)
I believe the answer is:
'visual aid' is the definition.
(a pair of pince-nez helps someone see)
'royal person not king backing doctrine' is the wordplay.
'royal person' becomes 'prince' (type of royal).
'not' is a deletion indicator.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'backing' says the letters should be written in reverse.
'doctrine' becomes 'zen' (Buddhist school or doctrine).
'prince' with 'r' removed is 'pince'.
'zen' backwards is 'nez'.
'gets' acts as a link.
(Other definitions for pince-nez that I've seen before include "Pair of glasses that clip on the nose, with no side-pieces" , "Lenses" , "Glasses clipped on to the nose" , "Clip-on glasses" , "Spectacles clipped to the nose" .)