Used to be binding, only disused (8)
I believe the answer is:
obsolete
'disused' is the definition.
(synonyms)
'used to be binding only' is the wordplay.
'used' is an anagram indicator.
'binding' is an insertion indicator.
'only' becomes 'sole' (synonyms).
'to'+'be'='tobe'
'tobe' is an anagram of 'obte'.
'obte' placed around 'sole' is 'OBSOLETE'.
(Other definitions for obsolete that I've seen before include "No longer useful" , "Discontinued" , "Outdated" , "Antiquated" , "Pass" .)