Old official carrying note — it’s all gas (7)
I believe the answer is:
methane
'all gas' is the definition.
'methane' can be an answer for 'gas' (I've seen this before). I'm unsure of the 'all' bit.
'old official carrying note it's' is the wordplay.
'old official' becomes 'than' (I can't justify this - if you can you should believe this answer much more).
'carrying' says to put letters next to each other (in a down clue, the bottom letters carry others).
'note' becomes 'me'.
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'than' after 'me' is 'methan'.
'methan'+'e'='METHANE'
Can you help me to learn more?
(Other definitions for methane that I've seen before include "Get fuel" , "Main constituent of natural gas" , "contributor to global warming" , "Combustible atmosphere" , "Firedamp" .)