"Hold on!” (4,2)
'hold on' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for wait that I've seen before include "Serve at table" , "Watchman" , "Delay, stand by" , "Be patient" , "Bide one's time" .)
(Another definition for wait up that I've seen is " Don't go to bed".)