On holiday, a king of Mercia (4)
I believe the answer is:
offa
'king of mercia' is the definition.
(I've seen this before)
'on holiday a' is the wordplay.
'on holiday' becomes 'off' (I've seen this before).
'off'+'a'='OFFA'
(Other definitions for offa that I've seen before include "Mercian dykemaker" , "King of Mercia, d. 796, with a dyke named after him" , "One-time ruler" , "Eighth century king of Mercia" , "Mercia's dyke builder" .)