Children's author left in pit (5)
I believe the answer is:
milne
'children's author' is the definition.
(AA Milne)
'left in pit' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'pit' becomes 'mine' (I've seen this before).
'l' inserted into 'mine' is 'MILNE'.
(Other definitions for milne that I've seen before include "A.A. -, Pooh author" , "Winnie-the-Pooh author" , "Creator of Eeyore" , "Children's author" , "AA man" .)