Laborious work framing paintings (8)
I believe the answer is:
toilsome
'laborious' is the definition.
(I know that laborious can be written as toilsome)
'work framing paintings' is the wordplay.
'work' becomes 'tome' (tome is a kind of work).
'framing' means one lot of letters goes inside another (inserted letters are framed by the outside letters).
'paintings' becomes 'oils' (oil paintings).
'tome' enclosing 'oils' is 'TOILSOME'.
(Other definitions for toilsome that I've seen before include "Needing much effort" , "needing much work" , "laborious" .)