Vibrating part of some instruments (4)
I believe the answer is:
reed
'vibrating part of some instruments' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reed that I've seen before include "'It vibrates in a clarinet, say (4)'" , "Type of musical instrument" , "Grass of marshes" , "Oliver perhaps" , "Tall water-loving grass" .)