Heavy work, including some art, is laborious (8)
I believe the answer is:
toilsome
'is laborious' is the definition.
(synonyms)
'heavy work including some art' is the wordplay.
'heavy work' becomes 'tome' (large book).
'including' is an insertion indicator.
'some art' becomes 'oils' (I've seen this in another clue).
'tome' enclosing 'oils' is 'TOILSOME'.
(Other definitions for toilsome that I've seen before include "laborious" , "needing much work" , "Needing much effort" .)