Setter would, in private, terrify (10)
I believe the answer is:
intimidate
'terrify' is the definition.
(thesaurus)
'setter would in private' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'would' becomes 'd' (I've seen this in other clues).
'in' indicates putting letters inside.
'private' becomes 'intimate' ('intimate' can be similar in meaning to 'private'**).
'i'+'d'='id'
'id' put into 'intimate' is 'INTIMIDATE'.
(Other definitions for intimidate that I've seen before include "Scare" , "Cow" , "Bully" , "Frighten" , "Threaten" .)