What's the subject of groan from hatemonger? (5)
I believe the answer is:
Here is my best explanation:
This clue might be a double definition.
'what's the subject' is the first definition.
('theme' can be a synonym of 'subject')
'groan from hatemonger?' is the second definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'of' is the link.
(Other definitions for theme that I've seen before include "Idea", "A subject or repeated melody", "Leitmotif", "Motif or melody", "essay", "Topic, leitmotif", "Matter", "issue", "Musical idea; topic" and "Motif - 4 across".)