Like the supreme ruler with the ultimate in sumptuous loose attire (8)
I believe the answer is:
overalls
'loose attire' is the definition.
'like the supreme ruler with the ultimate in sumptuous' is the wordplay.
I cannot quite understand how this works, but
'ruler' could be 'er' (abbreviation for Elizabeth Regina) and 'er' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for overalls that I've seen before include "Save roll (anag.)" , "Boilersuits" , "Dungarees" , "Protective wear" , "Protective garment" .)