Not new in the least! (5)
I believe the answer is:
stale
'not new in' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'least' is the wordplay.
'least' can be anagrammed to 'STALE'.
Though, I am unsure how this is indicated.
'the' is the link.
Can you help me to learn more?
(Other definitions for stale that I've seen before include "Boring" , "Least (anag.)" , "Old; musty" , "Musty - out of date" , "no longer bearing interest?" .)