UK Prime Minister's room left in a mess (11)
I believe the answer is:
chamberlain
'uk prime' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'minister's room left in a mess' is the wordplay.
'minister's room' becomes 'chamber' (I can't explain this - if you can you should give a lot more credence to this answer).
'left' becomes 'L' (common abbreviation).
'mess' indicates an anagram.
'in'+'a'='ina'
'ina' with letters rearranged gives 'ain'.
'chamber'+'l'+'ain'='CHAMBERLAIN'
Can you help me to learn more?
(Other definitions for chamberlain that I've seen before include "Prime minister, 1937-40" , "corporation's treasurer" , "Officer in charge of royal household" , "Court official - former prime minister" , "Manager of royal household - censorious lord" .)