Driver's encouraging utterance, injecting a gas (10)
I believe the answer is:
charioteer
'driver's' is the definition.
(charioteer is a kind of driver)
'encouraging utterance injecting a gas' is the wordplay.
'encouraging utterance' becomes 'cheer' (I am not sure about this - if you are sure you should believe this answer much more).
'injecting' indicates putting letters inside.
'gas' becomes 'riot' (I've seen this in another clue).
'a'+'riot'='ariot'
'cheer' enclosing 'ariot' is 'CHARIOTEER'.
Can you help me to learn more?
(Other definitions for charioteer that I've seen before include "Driver of horse-drawn fighting vehicle" , "Hippodrome racer" , "Her erotica (anag)" , "Driver common in early warfare" , "Porphyrius perhaps" .)