Lower organ, heading off vibration (5)
I believe the answer is:
udder
'lower organ' is the definition.
(cow organ - low can mean to make a mooing sound)
'heading off vibration' is the wordplay.
'heading off' means to remove the first letter.
'vibration' becomes 'judder' (I have seen 'Strong vibration' mean 'judder' so perhaps 'vibration' could also mean 'judder').
'judder' with its initial letter removed is 'UDDER'.
(Other definitions for udder that I've seen before include "milk supplier" , "Part of a cow" , "Cow's milk source" , "Teat of a cow" , "Cow's teat" .)