Notice rep's unusual greeting (9)
I believe the answer is:
reception
'greeting' is the definition.
(reception is a kind of greeting)
'notice rep's unusual' is the wordplay.
'unusual' indicates an anagram.
'notice'+'rep'='noticerep'
'noticerep' is an anagram of 'RECEPTION'.
(Other definitions for reception that I've seen before include "drinks party" , "Entrance for office" , "at home" , "How one is greeted" , "Formal function" .)