Deplore unconvincing book collection (6)
I believe the answer is:
lament
'deplore' is the definition.
(synonyms)
'unconvincing book collection' is the wordplay.
'unconvincing' becomes 'lame' (I've seen this in another clue).
'book collection' becomes 'NT' (New Testament).
'lame'+'nt'='LAMENT'
(Other definitions for lament that I've seen before include "Song of sorrow" , "Mourning song or poem" , "Mourn in mental way (6)" , "requiem" , "Keen" .)