A party piece? (7)
I believe the answer is:
faction
'a party' is the definition.
(thesaurus)
'piece?' is the wordplay.
I cannot quite see how this works, but
'piece?' could be 'fact' (I have seen 'Piece of information' mean 'fact' so perhaps 'piece' could also mean 'fact') and 'fact' is present in the answer.
The remaining letters 'ion' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for faction that I've seen before include "Act on if for a small group within a larger one" , "Dissenting group within a larger one" , "Wing" , "Minority who dissents from main group" , "part of party" .)