He is at the back of the room (5)
I believe the answer is:
denis
'he' is the definition.
(I've seen this before)
'is at the back of the room' is the wordplay.
'at the back of' says to put letters next to each other.
'the room' becomes 'den' (den is a kind of room).
'is' put after 'den' is 'DENIS'.
(Other definitions for denis that I've seen before include "Patron saint of France" , "St -, first Paris bishop" , "- Healey, former Chancellor of the Exchequer" .)