Put the phone down (4,2)
I believe the answer is:
'put the phone down' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hang up that I've seen before include "Modern term for emotional inhibition - get off the phone" , "Put on peg; disconnect call" , "Close call" , "In slang, an emotional inhibition - suspend it" , "Thing to suspend" .)