Slag off deserving author (10)
I believe the answer is:
galsworthy
'author' is the definition.
(John Galsworthy, 1867-1933)
'slag off deserving' is the wordplay.
'off' indicates an anagram.
'deserving' becomes 'worthy' (similar in meaning).
'slag' is an anagram of 'gals'.
'gals'+'worthy'='GALSWORTHY'
(Other definitions for galsworthy that I've seen before include "Writer of "The Forsyte Saga"" , "Playwright" , "Author of the Forsyte Saga" , "English dramatist" .)