Jacket's back on reference book after much delay (2,4)
I believe the answer is:
at last
'after much delay' is the definition.
(both can mean 'eventually')
'jacket's back on reference book' is the wordplay.
'back' suggests the final letters.
'on' means one lot of letters go next to another.
'reference book' becomes 'atlas' (atlas is a kind of reference book).
The last letter of 'jacket' is 't'.
't' after 'atlas' is 'AT LAST'.
(Other definitions for at last that I've seen before include "working as a cobbler?" , "'In the end (2,4)'" , "Finally, having waited a long time" , "Finally, after much delay" , "Eventually" .)