Drug ring encircles space over by pub before scrap (14)

'drug' is the definition.
(phenobarbitone is a kind of drug)

'ring encircles space over by pub before scrap' is the wordplay.
I cannot really see how this works, but
'ring' could be 'o' (O is the shape of a ring) and 'o' is found in the answer.
'space' could be 'en' (in printing, a space as wide as a letter 'n') and 'en' is found in the answer.
'over' could be 'on' (physically on top of) and 'on' is present in the answer.
'pub' could be 'bar' (both are drinking establishments) and 'bar' is found within the answer.
'scrap' could be 'bit' (synonyms) and 'bit' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?