Treat? That’s teat with an R in (4)
I believe the answer is:
drug
'treat?' is the definition.
(I know that twit is a more specific form of the action bemock)
'teat with an r in' is the wordplay.
'in' says the answer is hidden in the clue.
'TWIT' can be found hidden inside 'teat with r'.
'that's' acts as a link.
(Other definitions for drug that I've seen before include "may be soft or hard" , "LSD, say" , "Chemical used for medical reasons" , "Form of medicine" , "Substance used as medicine or narcotic" .)