Refuse Mercian king at length (5)
I believe the answer is:
offal
'refuse' is the definition.
(I've seen this before)
'mercian king at length' is the wordplay.
'mercian king' becomes 'offa' (I've seen this before).
'at' means one lot of letters go next to another.
'length' becomes 'l' (abbreviation in geometry).
'offa'+'l'='OFFAL'
(Other definitions for offal that I've seen before include "Less valuable edible parts of animal" , "Meaty waste" , "Entails and organs of animals, rubbish" , "Bits of waste meat" , "Entrails/organs as food" .)