Party debts are gruelling (9)
I believe the answer is:
laborious
'gruelling' is the definition.
(synonyms)
'party debts' is the wordplay.
'party' becomes 'labor' (Labor Party, Australian political party).
'debts' becomes 'IOUs'.
'labor'+'ious'='LABORIOUS'
'are' acts as a link.
(Other definitions for laborious that I've seen before include "Involving hard work" , "Using much time and effort" , "Gruelling" , "Strenuous" , "Arduous" .)