Tries somehow to interrupt a member of the family making pronouncement (9)
I believe the answer is:
assertion
'pronouncement' is the definition.
(thesaurus)
'tries somehow to interrupt a member of the family' is the wordplay.
'somehow' indicates an anagram.
'to interrupt' means one lot of letters goes inside another.
'member of the family' becomes 'son' (I've seen this in other clues).
'tries' is an anagram of 'serti'.
'a'+'son'='ason'
'serti' going within 'ason' is 'ASSERTION'.
'making' acts as a link.
(Other definitions for assertion that I've seen before include "Emphatic declaration" , "Claim" , "Confident declaration" , "Notarises (anag)" , "Confident statement of fact" .)