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 river' becomes 'tyne' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'containing' is an insertion indicator.
'half of' means to take half the letters.
'leak' with half the letters taken is 'le'.
'tyne' placed around 'le' is 'tylene'.
'ace'+'tylene'='ACETYLENE'
Can you help me to learn more?
(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?" .)