Slit releasing hydrogen or helium? (3)
I believe the answer is:
gas
'helium?' is the definition.
(helium is a kind of gas)
'slit releasing hydrogen' is the wordplay.
'slit' becomes 'gash' (I've seen this in another clue).
'releasing' indicates named letters should be taken away.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'gash' with 'h' taken out is 'GAS'.
'or' is the link.
(Other definitions for gas that I've seen before include "Eg, hydrogen" , "may fill American car" , "Form of fossil fuel" , "natural resource" , "Petrol (US)" .)