Finally producing a book of maps on time (2,4)
I believe the answer is:
at last
'finally' is the definition.
(synonyms)
'a book of maps on time' is the wordplay.
'a book of maps' becomes 'atlas' ('atlas' can be a synonym of 'book of maps').
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'atlas'+'t'='AT LAST'
'producing' is the link.
(Other definitions for at last that I've seen before include "Finally, after a long wait" , "'In the end (2,4)'" , "Finally, ultimately" , "In the end, after much delay" , "Finally, having waited a long time" .)