Spare hydrogen stored in container (4)
I believe the answer is:
thin
'spare' is the definition.
('thin' can be similar in meaning to 'spare')
'hydrogen stored in container' is the wordplay.
'hydrogen' becomes 'h' (H is the chemical symbol for hydrogen).
'stored in' is an insertion indicator.
'container' becomes 'tin' (tin is a kind of container).
'h' placed within 'tin' is 'THIN'.
(Other definitions for thin that I've seen before include "Meagre" , "Lean; narrow" , "Scrawny" , "Watered down" , "Flimsy" .)