Tries somehow to interrupt a member of the family making pronouncement (9)
I believe the answer is:
Here is my best explanation:
'pronouncement' is the definition.
'tries somehow to interrupt a member of the family' is the subsidiary indication.
'somehow' indicates an anagram.
'to interrupt' is an insertion indicator.
'member of the family' becomes 'son' (I've seen this in other clues).
'tries' is an anagram of 'serti'.
'serti' placed into 'ason' is 'assertion'.
'making' is the link.
(Other definitions for assertion that I've seen before include "Claim", "Confident declaration", "Declaration", "Statement", "Emphatic declaration", "Notarises (anag)" and "Confident statement of fact".)