Previously unknown (7-2)
I believe the answer is:
unheard-of
'previously unknown' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unheard-of that I've seen before include "Previously unknown" , "Without precedent" , "Not previously known or done - inaudible?" , "Unknown - novel" , "Fear hound being not previously known" .)