The whole of the room, except Henry (3)
I believe the answer is:
all
'the whole of' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for all that I've seen before include "nothing more" , "Passage in a formal garden" , "None excluded" , "Everybody; everything" , "Garden passage between trees or bushes" .)