Old boy on cocaine in swank bar (8)
I believe the answer is:
obstruct
'bar' is the definition.
(thesaurus)
'old boy on cocaine in swank' is the wordplay.
'old boy' becomes 'ob' (abbreviation).
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'cocaine' becomes 'c' (drugs abbreviation).
'in swank' becomes 'strut' (I can't explain this - if you can you should give a lot more credence to this answer).
'c' placed within 'strut' is 'struct'.
'ob'+'struct'='OBSTRUCT'
Can you help me to learn more?
(Other definitions for obstruct that I've seen before include "Impede - block" , "Hinder or impede" , "Prevent or hinder movement" , "Stymie" , "Block the passage or shut out the view" .)