Author is penning book in French (5)
I believe the answer is:
ibsen
'author' is the definition.
(Ibsen is an example)
'is penning book in french' is the wordplay.
'penning' indicates putting letters inside.
'book' becomes 'b' (abbreviation for book).
'in french' becomes 'en' (French word for 'in').
'is' going around 'b' is 'ibs'.
'ibs'+'en'='IBSEN'
(Other definitions for ibsen that I've seen before include "Norwegian playwright (Peer Gynt)" , "6 24 113 [SCAN-DIN-AVIAN] writer" , "Norwegian dramatist and poet" , "not one for 'All work and no play ...'" , "Author of The Master Builder, d. 1906" .)