For neuralgia, I dropped ecstasy - it's a gas (7)
I believe the answer is:
propane
'it's a gas' is the definition.
(propane is a kind of gas)
'for neuralgia i dropped ecstasy' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'neuralgia' becomes 'pain' (I've seen this in another clue).
'dropped' suggests deleting specific letters.
'ecstasy' becomes 'E' (informal term for the drug).
'pain' with 'i' taken out is 'pan'.
'pro'+'pan'+'e'='PROPANE'
(Other definitions for propane that I've seen before include "Bottle gas" , "A hydrocarbon gas" , "Type of gas" , "gaseous fuel" , "Bottled fuel gas" .)