Unique individual no longer available (3-3)
I believe the answer is:
one-off
'unique' is the definition.
(I've seen this before)
'individual no longer available' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'no longer available' becomes 'off' (I've seen this before).
'one'+'off'='ONE-OFF'
(Other definitions for one-off that I've seen before include "Unrepeatable experience" , "Not repeated or reproduced, a single thing" , "Unique thing" , "Done a single time" , "Unique person or occurrence" .)