Dirty work that has to be done to protect Society (5)
I believe the answer is:
dusty
'dirty' is the definition.
(similar in meaning)
'work that has to be done to protect society' is the wordplay.
'work that has to be done' becomes 'duty'.
'to protect' means one lot of letters goes inside another.
'society' becomes 's' (common abbreviation in names of organisations).
'duty' enclosing 's' is 'DUSTY'.
(Other definitions for dusty that I've seen before include "Needing cleaning" , "Boring" , "Powdery" , "Study (anag.)" , "Covered in fine particles" .)