Speak badly of drug, taken regularly in tiny amount (7)
I believe the answer is:
traduce
'speak badly of' is the definition.
(I've seen this before)
'drug taken regularly in tiny amount' is the wordplay.
'taken regularly' means one should take alternating letters.
'in' is an insertion indicator.
'tiny amount' becomes 'trace' (I've seen this before).
The alternate letters of 'drug' are 'du'.
'du' placed into 'trace' is 'TRADUCE'.
(Other definitions for traduce that I've seen before include "Asperse" , "Tell lies about (someone)" , "abuse" , "Defame, slander" , "Calumniate" .)