Gas expert on river containing half of leak (9)
I believe the answer is:
acetylene
'gas' is the definition.
(acetylene is a gas)
'expert on river containing half of leak' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'on' means one lot of letters go next to another.
'river' becomes 'tyne' (river in the North East of England).
'containing' means one lot of letters goes inside another.
'half of' suggests halving the letters.
'leak' halved is 'le'.
'tyne' placed around 'le' is 'tylene'.
'ace'+'tylene'='ACETYLENE'
(Other definitions for acetylene that I've seen before include "Illuminating stuff also used for joining" , "powers industrial torch" , "Gas used chiefly in welding" , "Flammable gas" , "cutting gas emission?" .)