Finally producing book of maps on time (2,4)
I believe the answer is:
at last
'finally' is the definition.
(synonyms)
'book of maps on time' is the wordplay.
'book of maps' becomes 'atlas' (synonyms).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'atlas'+'t'='AT LAST'
'producing' acts as a link.
(Other definitions for at last that I've seen before include "Eventually" , "None too soon" , "Where a snob worked" , "working as a cobbler?" , "Finally, after much delay" .)