Man, deadly, sitting next to monarch (6)
I believe the answer is:
feller
'man' is the definition.
(I've seen this before)
'deadly sitting next to monarch' is the wordplay.
I cannot quite see how this works, but
'deadly' could be 'fell' (I've seen this before) and 'fell' is present in the answer.
'monarch' could be 'er' (abbreviation for Elizabeth Regina, British Queen) and 'er' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for feller that I've seen before include "the cause of planes crashing?" , "Chap informally" , "Man casually" , "employed in forestry" , "Bloke - lumberjack" .)