Topic covered by Rotherhithe Messenger (5)
I believe the answer is:
theme
'topic' is the definition.
(I know that topic can be written as theme)
'covered by rotherhithe messenger' is the wordplay.
'covered by' indicates the answer is hidden within the clue.
'THEME' can be found hidden inside 'rotherhithe messenger'.
(Other definitions for theme that I've seen before include "Pervading idea in art" , "He met (anag) - subject" , "Subject matter, topic" , "Motif, topic" , "A subject or repeated melody" .)