Vet in plain spectacles (7)
I believe the answer is:
inspect
'vet' is the definition.
(I've seen this before)
'in plain spectacles' is the wordplay.
'in' indicates a hidden word.
'INSPECT' can be found hidden inside 'plain spectacles'.
(Other definitions for inspect that I've seen before include "Ecamine closely" , "Look critically at" , "Carefully examine" , "vet" , "Study" .)