Very hard worker is left inside bar (5)
I believe the answer is:
slave
'very hard worker' is the definition.
'left inside bar' is the wordplay.
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'bar' becomes 'save' (save is a kind of bar**).
'l' put inside '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" .)