Military vehicle hummed, emitting sulphur (4)
I believe the answer is:
tank
'military vehicle' is the definition.
(tank is a kind of military vehicle)
'hummed emitting sulphur' is the wordplay.
'hummed' becomes 'stank' (hum can mean to stink).
'emitting' suggests deleting specific letters.
'sulphur' becomes 'S' (synonyms).
'stank' with 's' taken away is 'TANK'.
(Other definitions for tank that I've seen before include "Armour-plated military vehicle" , "Car fuel container" , "Large storage chamber" , "Cistern - armoured vehicle" , "Water reservoir" .)