Knocked back dope snatching a watch (5)
I believe the answer is:
guard
'watch' is the definition.
(guarding is a kind of watching)
'knocked back dope snatching a' is the wordplay.
'knocked back' says the letters should be written backwards.
'dope' becomes 'drug'.
'snatching' means one lot of letters goes inside another.
'drug' going around 'a' is 'draug'.
'draug' written backwards gives 'GUARD'.
(Other definitions for guard that I've seen before include "Shield" , "Protection" , "state of caution" , "Warden; defend" , "Sentry" .)