Wait (4,4,4)
'wait' is the definition.
(hanging on is a kind of waiting)
This is the entire clue.
(Other definitions for hang on that I've seen before include "Don't let go - wait a minute" , "Hold tight; wait" , "Don't let go - just a mo'" , "Wait a mo', or hold tightly in suspense" , "Hold the line" .)
(Another definition for amo that I've seen is " - - , amas, amat".)
(Another definition for bide ones time that I've seen is " Await a favourable moment".)