Go berserk quietly after sheep turn grey (7)
I believe the answer is:
rampage
'go berserk' is the definition.
(I've seen this before)
'quietly after sheep turn grey' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'after' is a charade indicator (letters next to each other).
'sheep' becomes 'ram' (male sheep).
'turn grey' becomes 'age' (turning grey is a sign of aging).
'p' put after 'ram' is 'ramp'.
'ramp'+'age'='RAMPAGE'
(Other definitions for rampage that I've seen before include "Frenzy" , "Charge around being wild and destructive" , "Violent action" , "Uncontrolled rush" , "Storm" .)