Quiet beer possibly source of gas (5)
I believe the answer is:
shale
'source of gas' is the definition.
(gas can be extracted from shale rock)
'quiet beer' is the wordplay.
'quiet' becomes 'sh' ('sh' is used to tell someone to be quiet).
'beer' becomes 'ale' (type of beer).
'sh'+'ale'='SHALE'
'possibly' is the link.
(Other definitions for shale that I've seen before include "Oil-bearing rock" , "Sedimentary clay rock, which may contain oil" , "Stone that splits easily into fine pieces" , "Layered sedimentary rock" , "Soft sedimentary rock" .)