Studies elementary component of fuel (6)
I believe the answer is:
diesel
'fuel' is the definition.
(diesel fuel is a kind of fuel)
'studies elementary component' is the wordplay.
'component' indicates a hidden word (I've seen 'components' mean this (some components of the word make the answer)).
'DIESEL' is hidden in the letters of 'studies elementary'.
'of' acts as a link.
(Other definitions for diesel that I've seen before include "See lid for the engine" , "Vehicle fuel" , "Eponymous German engineer" , "Type of engine" , "Engine fuel" .)