Monopolise the laundry? Nonsense! (7)
I believe the answer is:
hogwash
'nonsense' is the definition.
(I've seen this before)
'monopolise the laundry?' is the wordplay.
'monopolise' becomes 'hog' (I've seen this before).
'the laundry?' becomes 'wash' ('wash' can be a synonym of 'laundry').
'hog'+'wash'='HOGWASH'
(Other definitions for hogwash that I've seen before include "Bunkum" , "Colloquial nonsense to pig cleanser" , "tripe" , "Utter nonsense" , "bull" .)