Much talk of carbon monoxide, say (3)
I believe the answer is:
gas
'much talk' is the definition.
'gas' can be an answer for 'talk' (gas can mean to chat). I am unsure of the 'much' bit.
'carbon monoxide say' is the wordplay.
'carbon monoxide' becomes 'g' (I can't explain this - if you can you should give a lot more credence to this answer).
'say' becomes 'as' (I've seen this in other clues).
'g'+'as'='GAS'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for gas that I've seen before include "It's not liquid or solid, like American petrol" , "Chatter (colloq.)" , "Fuel; idle talk" , "What's needed to drive" , "Hydrogen or methane, say" .)