Mars is missing hydrogen supplies (4)
I believe the answer is:
arms
'supplies' is the definition.
(arming is a kind of supplying)
'mars is missing hydrogen' is the wordplay.
'mars' becomes 'harms' (mar can mean to damage or harm).
'is missing' is a deletion indicator.
'hydrogen' becomes 'H' (chemical symbol).
'harms' with 'h' taken out is 'ARMS'.
(Other definitions for arms that I've seen before include "Blazonry; weaponry" , "International competition for weapons" , ". . . . and the Man (G B Shaw) (4)" , "pacifists won't take up" , "Provides with weapons" .)