Hold oneself in readiness (4)
(Other definitions for stand by that I've seen before include "the kind of passenger that may not get on with others" , "Act like supporters" , "last-minute ticket" , "Item or person ready for use in emergency" , "Back-up resource" .)
(Other definitions for wait that I've seen before include "Delay, stand by" , "Watchman" , "Delay action" , "Remain in expectation" , "singer" .)