Unique thing (3-3)
I believe the answer is:
one-off
'unique thing' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for one-off that I've seen before include "Unique thing" , "Not repeated - singular" , "Unique specimen" , "Not repeated or reproduced, a single thing" , "A single event, not reproduced or repeated" .)