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' indicates putting letters inside (some letters open a gap in others).
'one' placed around 'r' is 'orne'.
'os'+'b'+'orne'='OSBORNE'
'for' is the 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" .)