Certainly not behind revolving optical instrument (6)
I believe the answer is:
etalon
'optical instrument' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?
'certainly not behind revolving' is the wordplay.
'certainly not' becomes 'no'.
'behind' becomes 'late' (as in behind schedule).
'revolving' says the letters should be written in reverse.
'no'+'late'='nolate'
'nolate' reversed gives 'ETALON'.
Can you help me to learn more?
(Other definitions for etalon that I've seen before include "Spectroscopic device" , "measuring instrument" , "Reflecting device producing interfering light beams" .)