Brief very quietly delving into issue (7)
I believe the answer is:
apprise
'brief' is the definition.
(thesaurus)
'very quietly delving into issue' is the wordplay.
'very quietly' becomes 'pp' (musical abbreviation for pianissimo).
'delving into' means one lot of letters goes inside another.
'issue' becomes 'arise' (I am not sure about this - if you are sure you should believe this answer much more).
'pp' inserted into 'arise' is 'APPRISE'.
Can you help me to learn more?
(Other definitions for apprise that I've seen before include "Provide with information" , "Give notice" , "Give information to" , "Tell, advise" , "Formally inform, notify" .)