Lords look mature (7)
I believe the answer is:
peerage
'lords' is the definition.
(I've seen this before)
'look mature' is the wordplay.
'look' becomes 'peer' (to peer at something is to look at it).
'mature' becomes 'age' (synonyms).
'peer'+'age'='PEERAGE'
(Other definitions for peerage that I've seen before include "The nobility as a group" , "Dukes, earls, lords, ladies - the lot" , "Book listing nobles" , "Group of lords" , "Noble title and rank" .)