Trick giving up heroin for cocaine to persuade (4)
I believe the answer is:
coax
'persuade' is the definition.
(coaxing is a kind of persuading)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for coax that I've seen before include "Gradually persuade (someone)" , "Inveigle" , "attempt to influence" , "lead not to get wires crossed?" , "Flatter" .)