Mercian king taking most of entrails (4)
I believe the answer is:
offa
'mercian king' is the definition.
(I've seen this before)
'most of entrails' is the wordplay.
'most of' means to remove the last letter.
'entrails' becomes 'offal' (I have seen 'Edible entrails' mean 'offal' so perhaps 'entrails' could also mean 'offal').
'offal' with its final letter taken off is 'OFFA'.
'taking' acts as a link.
(Other definitions for offa that I've seen before include "King of Mercia, who may have built a dyke" , "One-time king" , "King of Mercia, who constructed earthwork on his Welsh border" , "King of Mercia, d. 796, with a dyke named after him" , "King of Mercia, 757-796" .)