Old queen's English hat mounted in part of UK (9)
I believe the answer is:
nefertiti
'old queen's' is the definition.
(ancient Egyptian queen)
'english hat mounted in part of uk' is the wordplay.
I cannot really understand how this works, but
'english' could be 'e' (abbreviation) and 'e' is present in the answer.
'part' could be 'tit' (tit is a kind of part) and 'tit' is located in the answer.
This explanation may well be incorrect...
(Other definitions for nefertiti that I've seen before include "Name of queen" , "Oddly, the Egyptian queen is an entire fit" , "old queen" , "Egyptian beauty queen" , "Beautiful queen who ruled Egypt with the Pharaoh" .)