Shout from old Methuselah interrupting quiet person (7)
I believe the answer is:
exclaim
'shout' is the definition.
(exclaiming is a kind of shouting)
'old methuselah interrupting quiet person' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'methuselah' becomes 'I' (where the crossword setter is 'Methuselah').
'interrupting' means one lot of letters goes inside another.
'quiet person' becomes 'clam' (I can't justify this - if you can you should give a lot more credence to this answer).
'i' put within 'clam' is 'claim'.
'ex'+'claim'='EXCLAIM'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for exclaim that I've seen before include "speak out boldly" , "Call" , "Declare" , "Cry out (in shock, etc)" , "say!" .)