A single trite quotation, under plain cover, all Jane’s Sir Walter would read? (10)
I believe the answer is:
baronetage
'a single trite quotation under plain cover all jane's sir walter would read?' is the definition.
I can't judge whether this definition defines the answer.
'single trite quotation under plain cover all jane's sir walter would read?' is the wordplay.
I cannot quite see how this works, but
'single' could be 'one' (I've seen this before) and 'one' is found in the answer.
'quotation' could be 'tag' (I've seen this before) and 'tag' is found within the answer.
'plain' could be 'bare' ('bare' can be similar in meaning to 'plain') and 'bare' is present in the remaining letters.
No letters remain.
This explanation may well be incorrect...
(Other definitions for baronetage that I've seen before include "Book with many titles" , "collection of titles" , "It lists title-holders" .)