Engineer can request a book, needing to start afresh, it's said (4,2,6,3)
I believe the answer is:
back to square one
'needing to start afresh it's said' is the definition.
(going back to square one means starting again)
'engineer can request a book' is the wordplay.
'engineer' indicates anagramming the letters.
'can'+'request'+'a'+'book'='canrequestabook'
'canrequestabook' is an anagram of 'BACK TO SQUARE ONE'.
(Other definitions for back to square one that I've seen before include "having another go at solving 1 ac?" , "Having completed a circuit" , "Starting again" , "no further forward?" , "Ready to start all over again" .)