Old man lying about gas (7)
I believe the answer is:
propane
'gas' is the definition.
(I've seen this before)
'old man lying about' is the wordplay.
'old man' becomes 'pa' (both can mean father).
'lying' becomes 'prone' (I've seen this before).
'about' indicates putting letters inside.
'pa' put inside 'prone' is 'PROPANE'.
(Other definitions for propane that I've seen before include "Gas used as fuel" , "A hydrocarbon gas" , "Bottle gas" , "Type of gas" , "gaseous fuel" .)