This writer's entertaining a lady (3)
I believe the answer is:
may
'lady' is the definition.
'this writer's entertaining a' is the wordplay.
'this writer's' becomes 'my' (belonging to the person who set the crossword).
'entertaining' indicates putting letters inside.
'my' placed around 'a' is 'MAY'.
(Other definitions for may that I've seen before include "Hawthorn blossom" , "UK PM" , "Time" , "The Darling Buds of - (H. E. Bates)" , "The permissive month" .)