Extract from Thackeray, a dull writer (4)
I believe the answer is:
hack
'writer' is the definition.
(hack writer is a kind of writer)
'extract from thackeray a dull' is the wordplay.
'extract from' says the answer is hidden in the clue (I've seen this in other clues).
'HACK' is hidden in the letters of 'thackeray dull'.
(Other definitions for hack that I've seen before include "jade" , "Chop roughly - mediocre journalist" , "Cut - mediocre journalist" , "Chop at hired horse?" , "get information illegally" .)