Trouble in service for gas (5)
I believe the answer is:
radon
'for gas' is the definition.
(I've seen this before)
'trouble in service' is the wordplay.
'trouble' becomes 'ado' (both can mean bother or commotion).
'in' indicates putting letters inside.
'service' becomes 'RN' (abbreviation for Royal Navy).
'ado' placed within 'rn' is 'RADON'.
(Other definitions for radon that I've seen before include "Gaseous element, Rn" , "A radioactive gas, symbol Rn" , "gets no reaction" , "Element, symbol Rn" , "Naturally occurring inert gas" .)