Withdraws books probing core elements of Einsteinian physics (4)
I believe the answer is:
ebbs
'withdraws' is the definition.
(I've seen this in another clue)
'books probing core elements of einsteinian physics' is the wordplay.
I cannot really see how this works, but
'books' could be 'bb' (two abbreviations for book) and 'bb' is present in the answer.
'elements' could be 'es' (E is an example) and 'es' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ebbs that I've seen before include "Goes out like tide" , "fades away" , "Wanes" , "flags" , "Declines gradually" .)