Popular book seized by detectives returning (7)
I believe the answer is:
demotic
'popular' is the definition.
(synonyms)
'book seized by detectives returning' is the wordplay.
'book' becomes 'tome' (a tome is a book).
'seized by' means one lot of letters goes inside another.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'returning' shows that the letters should be reversed in order.
'tome' going inside 'cid' is 'citomed'.
'citomed' back-to-front is 'DEMOTIC'.
(Other definitions for demotic that I've seen before include "It'd come (anag) - of the common people" , "vulgar?" , "Vernacular - c.o.d. item (anag)" , "(Of language) used by ordinary people" , "Colloquial; of the people" .)