Parisian thanks leader of army in old kingdom (6)
I believe the answer is:
mercia
'old kingdom' is the definition.
(Anglo-Saxon kingdom)
'parisian thanks leader of army' is the wordplay.
'parisian thanks' becomes 'merci' (I've seen this in another clue).
'leader of' suggests taking the first letters.
The initial letter of 'army' is 'a'.
'merci'+'a'='MERCIA'
'in' acts as a link.
(Other definitions for mercia that I've seen before include "part of old England" , "Old English kingdom, once ruled by Offa" , "Kingdom of Middle England" , "Ancient region" , "Anglo-Saxon kingdom" .)