No Boer could possibly be a king! (6)
I believe the answer is:
oberon
'a king' is the definition.
(I've seen this before)
'no boer could possibly' is the wordplay.
'could possibly' indicates an anagram.
'no'+'boer'='noboer'
'noboer' is an anagram of 'OBERON'.
'be' is the link.
(Other definitions for oberon that I've seen before include "Spirited king" , "...[IN DREAM] with king" , "Shakespeare's fairy king" , "charming king" , "Titania's consort" .)