Concerned with class - that's unusual in a lord (7)
I believe the answer is:
ordinal
'concerned with class' is the definition.
(relating to an order or class in a system)
'unusual in a lord' is the wordplay.
'unusual' indicates an anagram.
'in'+'a'+'lord'='inalord'
'inalord' with letters rearranged gives 'ORDINAL'.
'that's' is the link.
(Other definitions for ordinal that I've seen before include "gives position in class" , "First, second, third etc." , "Denoting a numerical order in a series" , "Ranking number" , "Relating to series order" .)