To talk down to a person -- it is nasty (9)
I believe the answer is:
patronise
'to talk down to' is the definition.
(I've seen this before)
'a person it is nasty' is the wordplay.
'a person' becomes 'patron' (patron is a kind of person**).
'it' becomes 'e' ('e' can mean 'electronic' which is similar to 'IT').
'nasty' indicates an anagram.
'e'+'is'='eis'
'eis' anagrammed gives 'ise'.
'patron'+'ise'='PATRONISE'
(Other definitions for patronise that I've seen before include "champion" , "Frequent as customer" , "Give encouragement or treat with condescension" , "Sponsor" , "Finance" .)