Finally book foremost of trips (2,4)
I believe the answer is:
at last
'finally' is the definition.
('at last' can be a synonym of 'finally')
'book foremost of trips' is the wordplay.
'book' becomes 'atlas' (atlas is a kind of book**).
'foremost of' indicates taking the first letters.
The initial letter of 'trips' is 't'.
'atlas'+'t'='AT LAST'
(Other definitions for at last that I've seen before include "Finally, after much delay" , "working as a cobbler?" , "Where a snob worked" , "Finally, having waited a long time" , "In the end, after much delay" .)