He's in bother over this fellow (8)
I believe the answer is:
theodore
'fellow' is the definition.
Both the answer and definition are singular nouns.
Maybe they are linked in a way I don't understand?
'he's in bother over' is the wordplay.
'he' becomes 'Odo'.
'in' is an insertion indicator.
'bother over' becomes 'there' (I can't justify this - if you can you should believe this answer much more).
'odo' going inside 'there' is 'THEODORE'.
'this' acts as a link.
Can you help me to learn more?
(Other definitions for theodore that I've seen before include "Man" , "ex-president was named thus" , "... Roosevelt" .)