They abandoned belief in a position on seat changing (9)
I believe the answer is:
apostates
'they abandoned belief' is the definition.
The definition suggests an adverb but the answer is not.
'a position on seat changing' is the wordplay.
'position' becomes 'post' (synonyms).
'on' says to put letters next to each other.
'changing' indicates anagramming the letters.
'seat' with letters rearranged gives 'ates'.
'a'+'post'+'ates'='APOSTATES'
'in' is the link.
Can you help me to learn more?
(Other definitions for apostates that I've seen before include "Those who renounce a belief" , "People who have renounced former beliefs" .)