A firm grip on the throttle? (12)
I believe the answer is:
stranglehold
'a firm' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'grip on the throttle?' is the wordplay.
'grip' becomes 'hold' (synonyms).
'on' is a charade indicator (letters next to each other).
'the throttle?' becomes 'strangle' (I've seen this before).
'hold' put after 'strangle' is 'STRANGLEHOLD'.
Can you help me to learn more?
(Other definitions for stranglehold that I've seen before include "exclusive control?" , "Complete power over a situation" , "Inexorable grip" , "Total restriction" , "Star long held (anag) - complete power over a person" .)