Spare hydrogen in container (4)
I believe the answer is:
thin
'spare' is the definition.
(similar in meaning)
'hydrogen in container' is the wordplay.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'in' indicates putting letters inside.
'container' becomes 'tin' (tin is a kind of container).
'h' going within 'tin' is 'THIN'.
(Other definitions for thin that I've seen before include "Made of light material" , "Unconvincing" , "Sparse; not dense" , "Dilute; lean" , "Implausible" .)