Nasty teacher -- ah, grief! (9)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
heartache
'grief' is the definition.
(I've seen this before)
'nasty teacher ah' is the wordplay.
'nasty' is an anagram indicator.
'teacher'+'ah'='teacherah'
'teacherah' with letters rearranged gives 'HEARTACHE'.
(Other definitions for heartache that I've seen before include "Great sadness" , "Suffering" , "mental anguish!" , "Emotional anguish" , "it gives rise to anguish" .)