Hospital in transaction for source of gas (5)
I believe the answer is:
shale
'source of gas' is the definition.
(gas can be extracted from shale rock)
'hospital in transaction' is the wordplay.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'in' is an insertion indicator.
'transaction' becomes 'sale' (sales are a type of transaction).
'h' put into 'sale' is 'SHALE'.
'for' acts as a link.
(Other definitions for shale that I've seen before include "Stone that splits easily into fine pieces" , "Easily split layered rock" , "Clayey rock" , "Sedimentary clay rock, which may contain oil" , "Oil-rich rock" .)