King of Mercia primarily laying waste (5)
I believe the answer is:
offal
'waste' is the definition.
(thesaurus)
'king of mercia primarily laying' is the wordplay.
'king of mercia' becomes 'offa' (I've seen this before).
'primarily' suggests taking the first letters.
The initial letter of 'laying' is 'l'.
'offa'+'l'='OFFAL'
(Other definitions for offal that I've seen before include "Meat product" , "Refuse - edible entrails of animal" , "Edible heart, liver and tongue, for instance" , "Liver, lights, etc" , "Entrails used as food" .)