A disturbance cutting into happiness of old vehicle driver (10)
I believe the answer is:
charioteer
'old vehicle driver' is the definition.
(driver of an old vehicle)
'a disturbance cutting into happiness' is the wordplay.
'disturbance' becomes 'riot' (I've seen this before).
'cutting into' indicates putting letters inside.
'happiness' becomes 'cheer'.
'a'+'riot'='ariot'
'ariot' put inside 'cheer' is 'CHARIOTEER'.
'of' is the link.
(Other definitions for charioteer that I've seen before include "Hippodrome racer" , "chap driving old vehicle" , "Porphyrius perhaps" , "Old carriage driver" , "Driver of horse-drawn fighting vehicle" .)