Thomas, say, upsetting brute after party (7)
I believe the answer is:
doubter
'thomas' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I can't see?
'upsetting brute after party' is the wordplay.
'upsetting' indicates anagramming the letters.
'after' means one lot of letters go next to another.
'party' becomes 'do' (do is a kind of party).
'brute' is an anagram of 'ubter'.
'ubter' after 'do' is 'DOUBTER'.
'say' acts as a link.
Can you help me to learn more?
(Other definitions for doubter that I've seen before include "E.g. St Thomas" , "One who is not sure" , "one such as Thomas" .)