Worker in bar to become thin -- reduced stones within! (7)
I believe the answer is:
tapster
'worker in bar' is the definition.
(old-fashioned term for someone working in a pub)
'thin reduced stones within' is the wordplay.
'thin' becomes 'taper' (I've seen this before).
'reduced stones' becomes 'st' (I've seen this in other clues; 'reduced' asks for a short form).
'within' is an insertion indicator.
'taper' enclosing 'st' is 'TAPSTER'.
'to become' acts as a link.
(Other definitions for tapster that I've seen before include "Old barperson" , "Bar employee" , "Bartender once" , "Rat pest (anag.)" , "old barman" .)