Reference book gets bought, finally. Finally! (2,4)
I believe the answer is:
at last
'finally' is the definition.
(synonyms)
'reference book gets bought finally' is the wordplay.
'reference book' becomes 'atlas' (atlas is a kind of reference book).
'gets' is a charade indicator (letters next to each other).
'finally' says to take the final letters.
The final letter of 'bought' is 't'.
'atlas'+'t'='AT LAST'
(Other definitions for at last that I've seen before include "Finally, after a long wait" , "Finally, having waited a long time" , "None too soon" , "Finally, after much delay" , "In the end, after much delay" .)