13A - revolutionary fruit of radical author (3,7) I have R?D?A????T
20A - a possible booth in an exhibition of US literature (10) I have ??R??N???N
27A - Shakesperean inn associated with concern in Hamlet (10) I have P???E?S???
12D - pictures belonging to Dogberry and Verges (10) I have ??N???B?E?
So we know Porpentine means the S is wrong in 27a. If you scrub the down answer that gave you the S does that affect teh letters for 13a? My first thought was red currant although I don't know how it relates to radical author