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' is an insertion indicator.
'quiet person' becomes 'clam' (I am not sure about this - if you are sure you should believe this answer much more).
'i' going into '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!" .)