Regularly drop speed (prescribed) (5)
I believe the answer is:
dosed
'prescribed' is the definition.
(I've seen this in another clue)
'regularly drop speed' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'drop'+'speed'='dropspeed'
The alternating letters of 'dropspeed' are 'DOSED'.
(Other definitions for dosed that I've seen before include "Gave medicine to" , "Administered a drug to" , "Given shot" , "Administered a portion of medicine to" , "Given medicine" .)