Book first of tests after much delay (2,4)
I believe the answer is:
at last
'after much delay' is the definition.
(both can mean 'eventually')
'book first of tests' is the wordplay.
'book' becomes 'atlas' (atlas is a kind of book).
'first of tests' becomes 't' (1st letter of 'tests').
'atlas'+'t'='AT LAST'
(Other definitions for at last that I've seen before include "Where cobbler located" , "Finally, after much delay" , "Finally, ultimately" , "None too soon" , "Finally, after a long wait" .)