Dexter is good worker (5-4)
I believe the answer is:
right-hand
'dexter' is the definition.
(thesaurus)
'good worker' is the wordplay.
'good' becomes 'right' (synonyms).
'worker' becomes 'hand' (hand is a kind of worker).
'right'+'hand'='RIGHT-HAND'
'is' acts as a link.
(Other definitions for right-hand that I've seen before include "drive in a British vehicle" , "Principally relied on such" .)