Work by area (western half of Quebec) that's dull (6)
I believe the answer is:
opaque
'dull' is the definition.
(I've seen this before)
'work by area western half of quebec' is the wordplay.
'work' becomes 'op' (operation, work done in an operating theatre).
'by' means one lot of letters go next to another.
'area' becomes 'a' (maths abbreviation).
'western half of' indicates taking half (the western or left-hand half).
'quebec' halved is 'que'.
'op'+'a'+'que'='OPAQUE'
'that's' is the link.
(Other definitions for opaque that I've seen before include "Shady" , "Mysterious" , "Not translucent" , "Not transparent, light can't pass through" , "Not clearly expressed" .)