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 owned by someone", "Person who is the property of another", "One in bondage", "Person owned for work", "One bound to serve", "Person owned by another person", "Work excessively hard", "Person in bondage", "Person in thraldom" and "Unpaid worker".)