Writer is best selling English novelist initially (5)
I believe the answer is:
ibsen
'writer' is the definition.
(Ibsen is an example)
'is best selling english novelist initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'is best selling english novelist' is 'IBSEN'.
(Other definitions for ibsen that I've seen before include "Norse playwright" , "Norwegian playwright (Peer Gynt)" , "'Doll's House' playwright" , "See 7 Down" , "Playmaker" .)