Ain't (4)
I believe the answer is:
isnt
'ain't' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for isnt that I've seen before include "Is not (colloq.)" , "- - This A Lovely Day? (song)" , "In the present, not to be" , "Does not happen to be" , "Ain't" .)