Spot one like Rees-Mogg occupying grandiose residence (6)
I believe the answer is:
pimple
'spot' is the definition.
(I've seen this before)
'one like rees-mogg occupying grandiose residence' is the wordplay.
'one like rees-mogg' becomes 'MP' (Jacob Rees-Mogg is a British MP).
'occupying' indicates putting letters inside (inserted letters occupy or enter the word).
'grandiose residence' becomes 'pile' (pile can mean a large building).
'mp' placed inside 'pile' is 'PIMPLE'.
(Other definitions for pimple that I've seen before include "Inflamed skin spot" , "Small pustule" .)