Rascally plot — assign book to top shelf? (3-2,3)
I believe the answer is:
put-up job
'rascally plot' is the definition.
'assign book to top shelf?' is the wordplay.
'assign' becomes 'put' ('put' can be a synonym of 'assign').
'book' becomes 'upj' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to top' means to remove the first letter.
'shelf?' becomes 'hob' (hob is a kind of shelf).
'hob' with its first letter taken off is 'ob'.
'put'+'upj'+'ob'='PUT-UP JOB'
(Other definitions for put-up job that I've seen before include "Dishonest scheme" , "Prearranged scam" , "Craftily conceived undertaking" , "'Pre-arrangement in order to deceive (3,2,3)'" , "sham!" .)