Book of maps a sailor’s taken up (5)
I believe the answer is:
atlas
'book of maps' is the definition.
(synonyms)
'a sailor's taken up' is the wordplay.
'sailor' becomes 'salt' (salt is a kind of sailor).
'taken up' is a reversal indicator.
'salt' backwards is 'tlas'.
'a'+'tlas'='ATLAS'
(Other definitions for atlas that I've seen before include "Titan who held up the sky" , "N African mountains - seemaps book" , "Collection of maps" , "Mythological Titan (involved with the world of maps?)" , "Cartographer's work" .)