Explosive gas available primarily in hospital room (7)
I believe the answer is:
warhead
'explosive' is the definition.
(I know that warhead is a type of explosive)
'gas available primarily in hospital room' is the wordplay.
'gas' becomes 'he' (chemical symbol for Helium).
'primarily' says to take the initial letters.
'in' means one lot of letters goes inside another.
'hospital room' becomes 'ward' (I've seen this before).
The initial letter of 'available' is 'a'.
'he'+'a'='hea'
'hea' placed within 'ward' is 'WARHEAD'.
(Other definitions for warhead that I've seen before include "Explosive part of a weapon" , "Charge" , "Explosive missile part" , "bit of a weapon" , "Missile explosive unit" .)