Varnish ingredient from diesel emissions (5)
I believe the answer is:
elemi
'varnish ingredient' is the definition.
(I've seen this in another clue)
'from diesel emissions' is the wordplay.
'from' indicates a hidden word.
'ELEMI' can be found hidden inside 'diesel emissions'.
(Other definitions for elemi that I've seen before include "It's fragrance" , "Fragrant tree resin used as incense - I'm Lee (anag)" , "Resinous stuff" , "aromatic resin" , "Fragrant resin used as incense - Emile (anag)" .)