He wrote some lines - Bill's written back (5)
I believe the answer is:
ibsen
'he wrote' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'some lines bill's written back' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'back' shows that the letters should be reversed in order.
'nesbi' can be found hidden inside 'lines bills written'.
'nesbi' reversed gives 'IBSEN'.
Can you help me to learn more?
(Other definitions for ibsen that I've seen before include "Norse playwright" , ""Doll's House" dramatist" , "Henrik -, dramatist from fourteen down (Hedda Gabler)" , "Norwegian playwright, d. 1906" , "Norwegian dramatist, d. 1906" .)