Large book one Rex opens for dramatist (7)
I believe the answer is:
osborne
'dramatist' is the definition.
(I've seen this before)
'large book one rex opens' is the wordplay.
'large' becomes 'os' (abbreviation for oversize).
'book' becomes 'b' (abbreviation for book).
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'opens' is an insertion indicator (insertion requires an opening to be made).
'one' enclosing 'r' is 'orne'.
'os'+'b'+'orne'='OSBORNE'
'for' acts as a link.
(Other definitions for osborne that I've seen before include "John --, English dramatist" , "13 across's IOW house" , "Queen Victoria's IOW house" , "Chancellor" , "Look Back in Anger writer" .)