On the quiet, many take the plunge (3)
I believe the answer is:
dip
'the plunge' is the definition.
(I know that dip is a type of plunge)
'on the quiet many' is the wordplay.
'on' means one lot of letters go next to another.
'the quiet' becomes 'ip' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'many' becomes 'd'.
'ip' put after 'd' is 'DIP'.
'take' acts as a link.
Can you help me to learn more?
(Other definitions for dip that I've seen before include "A brief informal swim" , "An informal swim or a depression" , "A quick bathe" , "Pickpocket" , "Sink; immerse" .)