Sailor came up after a book of maps (5)
I believe the answer is:
atlas
'book of maps' is the definition.
(I know this)
'sailor came up after a' is the wordplay.
'sailor' becomes 'salt' (salt is a kind of sailor).
'came up' says the letters should be written in reverse (I've seen 'comes up' mean this).
'after' means one lot of letters go next to another.
'salt' back-to-front is 'tlas'.
'tlas' after 'a' is 'ATLAS'.
(Other definitions for atlas that I've seen before include "Sky-toting Titan" , "Geographical book" , "Titan who supported the heavens" , "Segment of spine" , "Large-format book" .)