Resinous substance contributing to travel emissions (5)
I believe the answer is:
elemi
'resinous substance' is the definition.
(elemi is a resin)
'contributing to travel emissions' is the wordplay.
'contributing to' says the answer is hidden in the clue (the answer contributes to part of the clue ).
'ELEMI' can be found hidden inside 'travel emissions'.
(Other definitions for elemi that I've seen before include "Extract" , "Fragrant tree resin used as incense - I'm Lee (anag)" , "Fragrant resin used as incense - Emile (anag)" , "Oleoresin" , "It's fragrance" .)