Rely on trapping hydrogen's driving force (6)
I believe the answer is:
thrust
'driving force' is the definition.
('thrust' can be a synonym of 'driving force')
'rely on trapping hydrogen's' is the wordplay.
'rely on' becomes 'trust' ('trust' can be a synonym of 'rely on').
'trapping' is an insertion indicator.
'hydrogen' becomes 'h' (H is the chemical symbol for hydrogen).
'trust' placed around 'h' is 'THRUST'.
(Other definitions for thrust that I've seen before include "Lunge forward" , "Force producing motion" , "The essence" , "Push forcefully" , "Push forward as with fencing sword" .)