Setter would put in hint to threaten (10)
I believe the answer is:
intimidate
'threaten' is the definition.
(I've seen this before)
'setter would put in hint' is the wordplay.
'setter' becomes 'i' (the person that set the crossword).
'would' becomes 'd' (I've seen this in other clues).
'put in' indicates putting letters inside.
'hint' becomes 'intimate' (intimating is a kind of hinting**).
'i'+'d'='id'
'id' going inside 'intimate' is 'INTIMIDATE'.
'to' acts as a link.
(Other definitions for intimidate that I've seen before include "Cow" , "Threaten" , "Bully" , "Scare" , "Strike fear into" .)