One departing bar is heard (6)
I believe the answer is:
Here is my best explanation:
'one departing' is the definition.
'bar is heard' is the subsidiary indication.
'bar' becomes 'lever' (lever is a kind of bar).
'is heard' indicates a 'sounds like' (homophone) clue.
'lever' sounds like 'leaver'.
(Other definitions for leaver that I've seen before include "One departing" and "Person departing".)