Old boy's yarn about cold bar (8)
I believe the answer is:
obstacle
'bar' is the definition.
(thesaurus)
'old boy's yarn about cold' is the wordplay.
'old boy's' becomes 'obs' (I've seen this in other clues).
'yarn' becomes 'tale' ('tale' can be a synonym of 'yarn').
'about' is an insertion indicator.
'cold' becomes 'c' (eg on taps).
'tale' placed around 'c' is 'tacle'.
'obs'+'tacle'='OBSTACLE'
(Other definitions for obstacle that I've seen before include "stumbling block" , "It obstructs or hinders" , "hold-up" , "Maybe let" , "Block to progress" .)