Old boy's story about opening of cocktail bar (8)
I believe the answer is:
obstacle
'bar' is the definition.
(thesaurus)
'old boy's story about opening of cocktail' is the wordplay.
'old boy's' becomes 'obs' (I've seen this in other clues).
'story' becomes 'tale' ('tale' can be a synonym of 'story').
'about' means one lot of letters goes inside another.
'opening of' says to take the initial letters.
The first letter of 'cocktail' is 'c'.
'tale' placed around 'c' is 'tacle'.
'obs'+'tacle'='OBSTACLE'
(Other definitions for obstacle that I've seen before include "Hurdle, impediment" , "Maybe let" , "Progress blocker" , "Stab Cleo (anag) - which hinders progress" , "The problem" .)