Tender-sounding old king? (4)
I believe the answer is:
offa
'old king?' is the definition.
(King of Mercia)
'tender-sounding' is the wordplay.
'tender' becomes 'offer' (synonyms).
'sounding' shows a homophone (sound like).
'offer' sounds like 'OFFA'.
(Other definitions for offa that I've seen before include "Mercian leader" , "Organiser of defence" , "One-time king" , "He created barrier" , "8th-century Mercian king" .)