NCO despatched to receive drug rehab at regular intervals (8)
I believe the answer is:
sergeant
'nco' is the definition.
(thesaurus)
'despatched to receive drug rehab at regular intervals' is the wordplay.
'despatched' becomes 'sent' (I've seen this before).
'to receive' is an insertion indicator.
'at regular intervals' means one should take alternating letters.
'drug'+'rehab'='drugrehab'
The alternating letters of 'drugrehab' are 'rgea'.
'sent' placed around 'rgea' is 'SERGEANT'.
(Other definitions for sergeant that I've seen before include "Military or police officer" , "Soldier and lover in Hardy novel" , "Gets near (anag) -- NCO" , "Pepper, say" , "Noncommissioned military officer" .)