Deplorable, the French historian's work (10)
I believe the answer is:
chronicles
'historian's work' is the definition.
'deplorable the french' is the wordplay.
'deplorable' becomes 'chronic' (I've seen this before).
'the french' becomes 'les' (les is the in french).
'chronic'+'les'='CHRONICLES'
(Other definitions for chronicles that I've seen before include "events listed by The Times" , "records" , "Accounts" , "OT books; Holinshed work" , "Book" .)