Blackbird's a new one (3)
I believe the answer is:
ani
'blackbird's' is the definition.
(I've seen this in another clue)
'a new one' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'one' becomes 'i' (Roman numeral).
'a'+'n'+'i'='ANI'
(Other definitions for ani that I've seen before include "Black bird" , "Indigo dye." , "Bird of the cuckoo family" , "One winged" , "Black cuckoo" .)