Old boy's story about opening of cellar bar (8)
I believe the answer is:
obstacle
'bar' is the definition.
(thesaurus)
'old boy's story about opening of cellar' 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' is an insertion indicator.
'opening of' suggests taking the first letters.
The first letter of 'cellar' is 'c'.
'tale' placed around 'c' is 'tacle'.
'obs'+'tacle'='OBSTACLE'
(Other definitions for obstacle that I've seen before include "Hindrance, obstruction" , "Block to progress" , "Barrier or deterrent" , "Blockage, hindrance" , "stumbling block" .)