Drug that's positive in origin (5)
I believe the answer is:
speed
'drug' is the definition.
(speed is a kind of drug)
'positive in origin' is the wordplay.
'positive' becomes 'p'.
'in' is an insertion indicator.
'origin' becomes 'seed' (seed is a kind of origin).
'p' placed within 'seed' is 'SPEED'.
'that's' is the link.
(Other definitions for speed that I've seen before include "Swiftness" , "Pace or tempo" , "Rate of progress" , "Rtae of progress" , "make rapid progress" .)