Spot member occupying mansion (6)
I believe the answer is:
pimple
'spot' is the definition.
(I've seen this before)
'member occupying mansion' is the wordplay.
'member' becomes 'MP' (Member of Parliament).
'occupying' indicates putting letters inside (inserted letters occupy or enter the word).
'mansion' becomes 'pile' (I've seen this in other clues).
'mp' put into 'pile' is 'PIMPLE'.
(Other definitions for pimple that I've seen before include "Inflamed skin spot" , "Small pustule" .)