Belgian city good, then bad (5)
I believe the answer is:
ghent
'belgian city' is the definition.
(I've seen this before)
'good then bad' is the wordplay.
'good' becomes 'g' (abbreviation).
'bad' is an anagram indicator.
'then' with letters rearranged gives 'hent'.
'g'+'hent'='GHENT'
(Other definitions for ghent that I've seen before include "Flanders city/port" , "City of northwest Belgium, known for its cloth industry" , "source of good news" , "Belgian port" , "Belgian city (from which the "good news" was brought to Aix)" .)