Veronica not well — drug appears (5)
I believe the answer is:
speed
'drug appears' is the definition.
I can't judge whether this defines the answer.
'veronica not well' is the wordplay.
'veronica' becomes 'speedwell' ('speedwell' can be a synonym of 'veronica').
'not' suggests deleting specific letters.
'speedwell' with 'well' removed is 'SPEED'.
(Other definitions for speed that I've seen before include "Hasten" , "Pace or tempo" , "Rate of progress" , "Dispatch" , "The going rate" .)