Stupid nit, not being polite, thrust in (7)
I believe the answer is:
intrude
'thrust in' is the definition.
(thesaurus)
'stupid nit not being polite' is the wordplay.
'stupid' indicates anagramming the letters (in a stupid order).
'not being polite' becomes 'rude' ('rude' is the opposite to 'polite').
'nit' with letters rearranged gives 'int'.
'int'+'rude'='INTRUDE'
(Other definitions for intrude that I've seen before include "Put forward" , "Enter uninvited and unwelcome" , "Because of temperature, blue" , "Push in unwantedly" , "Pry" .)