Prime assembly promises to be high-handed (9)
I believe the answer is:
imperious
'be high-handed' is the definition.
(I've seen this before)
'prime assembly promises' is the wordplay.
'assembly' is an anagram indicator.
'promises' becomes 'IOUs' (promises to pay a debt).
'prime' is an anagram of 'imper'.
'imper'+'ious'='IMPERIOUS'
'to' is the link.
(Other definitions for imperious that I've seen before include "Arrogant, domineering" , "high-handed" , "Bossy" , "Haughty - domineering" .)