Dad lying about gas (7)
I believe the answer is:
propane
'gas' is the definition.
(I know that propane is a type of gas)
'dad lying about' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'lying' becomes 'prone' (I've seen this before).
'about' indicates putting letters inside.
'pa' inserted inside 'prone' is 'PROPANE'.
(Other definitions for propane that I've seen before include "Bottled fuel gas" , "gaseous fuel" , "Gas used as fuel" , "Bottle gas" , "A hydrocarbon gas" .)