Seldom seen, king with deformed ear (4)
I believe the answer is:
rare
'seldom seen' is the definition.
(thesaurus)
'king with deformed ear' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'with' says to put letters next to each other.
'deformed' indicates an anagram.
'ear' anagrammed gives 'are'.
'r'+'are'='RARE'
(Other definitions for rare that I've seen before include "Meat that's lightly cooked" , "Seen once in a blue moon" , "early" , "few and far between" , "Exceptionable" .)