Block hydrogen being put into fuel (5)
I believe the answer is:
choke
'block' is the definition.
(choking is a kind of blocking)
'hydrogen being put into fuel' is the wordplay.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'being put into' is an insertion indicator.
'fuel' becomes 'coke' (coke is a kind of fuel).
'h' going inside 'coke' is 'CHOKE'.
(Other definitions for choke that I've seen before include "Prevent from getting air" , "Become speechless (with emotion)" , "clog" , "Asphyxiate" , "Strangle" .)