Units regularly taking drugs (4)
I believe the answer is:
ones
'units' is the definition.
(I know this)
'regularly taking drugs' is the wordplay.
'regularly taking' becomes 'on' (as in 'on drugs').
'drugs' becomes 'es' (ecstasy tablets).
'on'+'es'='ONES'
(Other definitions for ones that I've seen before include "Units" , "Singletons" , "(Of ship) returning to port of origin" , "Singles" , "1 & 1 & 1" .)