Person regularly taking tablet (3)
I believe the answer is:
one
'person' is the definition.
('one' can mean an unspecified person)
'regularly taking tablet' is the wordplay.
'regularly taking' becomes 'on' (as in 'on drugs').
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "Half of two" , "doesn't count for much" , "Single, unitary" , "Lowest cardinal number" , "Smallest whole number" .)