Crooner pens a line for an American author (8)
I believe the answer is:
salinger
'author' is the definition.
(JD Salinger)
'crooner pens a line for an american' is the wordplay.
'crooner' becomes 'singer' (I've seen this before).
'pens' is an insertion indicator.
'a line' becomes 'l'.
'for' means one lot of letters go next to another (I've seen this in other clues).
'an american' becomes 'a' (common abbreviation - e.g. in organisation names).
'l' put after 'a' is 'al'.
'singer' going around 'al' is 'SALINGER'.
(Other definitions for salinger that I've seen before include "J. D. ..., author of Catcher in the Rye" , "US novelist" , "'JD..., author of The Catcher in the Rye (8)'" , "Jerome D ........ wrote ` ` The Catcher in the Rye' '" , "US author of The Catcher in the Rye" .)