Will group put back unexciting books? (9)
I believe the answer is:
testament
'will' is the definition.
(synonyms)
'group put back unexciting books?' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'put back' shows that the letters should be reversed in order.
'unexciting' becomes 'tame' (similar in meaning).
'books?' becomes 'NT' (New Testament).
'set' in reverse letter order is 'tes'.
'tes'+'tame'+'nt'='TESTAMENT'
(Other definitions for testament that I've seen before include "books, with 1a [OLD]" , "Statement (anag.)" , "Earnest" , "Will or solemn declaration" , "Witness" .)