Fragrant resin found in diesel emissions (5)
I believe the answer is:
elemi
'fragrant resin' is the definition.
(I've seen this before)
'in diesel emissions' is the wordplay.
'in' indicates a hidden word.
'ELEMI' can be found hidden inside 'diesel emissions'.
'found' is the link.
(Other definitions for elemi that I've seen before include "Fragrant resin used as incense - Emile (anag)" , "Resinous stuff" , "Tree resin; ie, elm (anag.)" , "aromatic resin" , "It's fragrance" .)