Room in a box is restricting, extremely (10)
I believe the answer is:
terminally
'extremely' is the definition.
(I've seen this before)
'room in a box is restricting' is the wordplay.
'room' becomes 'rm' (abbreviation for room).
'box' becomes 'telly' (both can mean a television set).
'is restricting' is an insertion indicator.
'rm'+'in'+'a'='rmina'
'rmina' put within 'telly' is 'TERMINALLY'.
(Other definitions for terminally that I've seen before include "extremely" , "At the end" .)