The most common and most likely answer to this crossword clue is the 4 letter word OFFA
I've seen this crossword clue in The Guardian
O
F
F
A
More clues leading to the result OFFA
- Tender-sounding Mercian king?
- Historic defence beginning when king covered in woad?
- King is present to speak
- Tender, when speaking, for one for whom the earth moved?
- Organiser of defence to hold out for a hearing
- Old sovereign tender, by the sound of it?
- Start when Duke moving key defence installation (5,4
- Mercian king taking most of entrails
- King of Mercia once listened to proposal
- Refuse to part with pound for old ruler
Buddy explains!

The straight part
We know that "King of Mercia once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case king of mercia once = offa.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does OFFA mean?
OFFA preposition- Off of.