King of Mercia once listened to proposal (4)
I believe the answer is:
offa
'king of mercia once' is the definition.
(8th-century king of Mercia)
'listened to proposal' is the wordplay.
'listened to' indicates a 'sounds like' (homophone) clue.
'proposal' becomes 'offer' (I've seen this before).
'offer' is a homophone of 'OFFA'.
(Other definitions for offa that I've seen before include "King of Mercia, who constructed earthwork on his Welsh border" , "Mercian dykemaker" , "Organiser of defence" , "Old Mercia king" , "Earthwork maker" .)