Cut back duty originally capped by fuel regulator (9)
I believe the answer is:
throttled
'capped by fuel regulator' is the definition.
The definition and answer can be both related to change as well as being past participle verbs.
Maybe there's an association between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for throttled that I've seen before include "Suppressed" , "Choked" , "Strangled manually" .)