Bidding for a change in Thebes (6)
I believe the answer is:
behest
'bidding' is the definition.
(behest is a kind of bidding)
'change in thebes' is the wordplay.
'change in' is an anagram indicator.
'thebes' with letters rearranged gives 'BEHEST'.
'for a' acts as a link.
(Other definitions for behest that I've seen before include "Order" , "(At someone's) command" , "Command - Thebes (anag)" , "Command, bidding" .)