Detailed description of glasses? Not quite! (4)
I believe the answer is:
spec
'detailed description' is the definition.
(I've seen this before)
'glasses? not quite' is the wordplay.
'glasses?' becomes 'specs' ('specs' can be a synonym of 'glasses').
'not quite' means to remove the last letter.
'specs' with its last letter taken off is 'SPEC'.
'of' acts as a link.
(Other definitions for spec that I've seen before include "Detailed design document (abbr.)" , "Gamble (colloq.)" , "glasses?" , "estimation?" , "Required standard of workmanship" .)