Moderate, not entirely correct to break silence (6)
I believe the answer is:
medium
'moderate' is the definition.
(similar in meaning)
'not entirely correct to break silence' is the wordplay.
'not entirely' means to remove the last letter.
'correct' becomes 'edit' (I've seen this before).
'to break' means one lot of letters goes inside another.
'silence' becomes 'mum' (mum is a kind of silence).
'edit' with its last letter taken off is 'edi'.
'edi' placed inside 'mum' is 'MEDIUM'.
(Other definitions for medium that I've seen before include "Intermediary with the spirit world" , "Average; clairvoyant" , "Average or intermediate" , "Spiritualist who conducts seances" , "Mean sort of clairvoyant." .)