Very hard worker is left inside bar (5)
I believe the answer is:
Here is my best explanation:
'very hard worker' is the definition.
'left inside bar' is the subsidiary indication.
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'bar' becomes 'save' (save is a kind of bar**).
'l' placed within 'save' is 'slave'.
'is' is the link.
(Other definitions for slave that I've seen before include "Person who is the property of another", "Work excessively hard", "Person who is owned by someone", "Person in bondage", "Unpaid worker", "Person owned for work", "One bound to serve", "Person owned by another person", "One in bondage" and "Person in thraldom".)