Finally cut down book of maps (5)
I believe the answer is:
atlas
'book of maps' is the definition.
(synonyms)
'finally cut down' is the wordplay.
'finally' becomes 'at last' (synonyms).
'cut down' means to remove the last letter.
'atlast' with its last letter removed is 'ATLAS'.
(Other definitions for atlas that I've seen before include "Supporter of the classics" , "Book of maps - fifteenth satellite of Jupiter" , "Ballistic missile" , "Asian satin" , "Large-format book" .)