Banned doper worried about drug dose when injecting twice (10)
I believe the answer is:
prohibited
'banned' is the definition.
('prohibited' can be a synonym of 'banned')
'doper worried about drug dose when injecting twice' is the wordplay.
'worried' indicates anagramming the letters.
'about' is an insertion indicator.
'drug dose' becomes 'hit'.
'when injecting' means one lot of letters goes inside another.
'twice' becomes 'bi' (eg biannual, biweekly).
'doper' with letters rearranged gives 'proed'.
'hit' going around 'bi' is 'hibit'.
'proed' enclosing 'hibit' is 'PROHIBITED'.
(Other definitions for prohibited that I've seen before include "off-limits" , "Criminal" , "Banned" , "Forbidden" .)