He sets the speed for a heart stimulant (9)
I believe the answer is:
pacemaker
'he sets the' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'speed for a heart stimulant' is the wordplay.
'speed' becomes 'pace' (pace is a kind of speed).
'for' means one lot of letters go next to another.
'a heart stimulant' becomes 'maker' (I can't explain this - if you can you should believe this answer much more).
'pace'+'maker'='PACEMAKER'
Can you help me to learn more?
(Other definitions for pacemaker that I've seen before include "leader at the start" , "Cardiac device" , "Device inserted into chest to stimulate heart" , "Device to regulate the heartbeat" , "Electrical device put in heart to stimulate pulse" .)