Harry found her appalling at first — that’s without precedent (7-2)
I believe the answer is:
unheard-of
'without precedent' is the definition.
(I've seen this before)
'harry found her appalling at first' is the wordplay.
'harry' indicates an anagram (harry can mean to attack or spoil).
'at first' suggests taking the first letters.
The first letter of 'appalling' is 'a'.
'found'+'her'+'a'='foundhera'
'foundhera' anagrammed gives 'UNHEARD-OF'.
'that's' is the link.
(Other definitions for unheard-of that I've seen before include "Fear hound being not previously known" , "Without precedent" , "Previously unknown" , "Unknown - novel" , "Not previously known or done - inaudible?" .)