Harmony when worker's organisation accepts old Bob (6)
I believe the answer is:
unison
'harmony' is the definition.
(unison is a kind of harmony)
'worker's organisation accepts old bob' is the wordplay.
'worker's organisation' becomes 'union'.
'accepts' is an insertion indicator.
'old bob' becomes 's' (abbreviation for bob or shilling used before 1971).
'union' enclosing 's' is 'UNISON'.
'when' is the link.
(Other definitions for unison that I've seen before include "Close agreement" , "Sounding or singing together at the same pitch" , "There's agreement" , "All singing the same note" , "concert" .)