Finally book end of August (2,4)
I believe the answer is:
at last
'finally' is the definition.
(synonyms)
'book end of august' is the wordplay.
'book' becomes 'atlas' (atlas is a kind of book).
'end of' indicates one should take the final letters.
The final letter of 'august' is 't'.
'atlas'+'t'='AT LAST'
(Other definitions for at last that I've seen before include "Where cobbler located" , "In the end, after much delay" , "'In the end (2,4)'" , "Finally, after much delay" , "Finally, having waited a long time" .)