Possibly a last book (5)
I believe the answer is:
atlas
'book' is the definition.
(atlas is a kind of book)
'possibly a last' is the wordplay.
'possibly' indicates anagramming the letters.
'a'+'last'='alast'
'alast' with letters rearranged gives 'ATLAS'.
(Other definitions for atlas that I've seen before include "Ballistic missile" , "Titan compelled by Zeus to support the sky" , "Sky carrier - map book" , "Set of charts" , "collected world records?" .)