Prevent one with little hesitation inhaling initially untested gas (9)

'gas' is the definition.
(I know that Deuterium is a type of gas)

'prevent one with little hesitation inhaling initially untested' is the wordplay.
'prevent' becomes 'deter' (I've seen this before).
'one with' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'little hesitation' becomes 'um'.
'inhaling' indicates putting letters inside.
'initially' says to take the initial letters.
The first letter of 'untested' is 'u'.
'deter'+'i'+'um'='deterium'
'deterium' enclosing 'u' is 'DEUTERIUM'.