Tartan Army held back by copper dispersing a party in Cardiff (5,5)
I believe the answer is:
plaid cymru
'party in cardiff' is the definition.
'plaid cymru' can be an answer for 'party' (Welsh political party). I am not sure about the 'in cardiff' bit.
'tartan army held back by copper dispersing a' is the wordplay.
I cannot quite understand how this works, but
'tartan' could be 'plaid' (synonyms) and 'plaid' is located in the answer.
The remaining letters 'cymru' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for plaid cymru that I've seen before include "Welsh Nationalist party" , "party in Wales" , "party in UK" , "Welsh political party" .)