Stop Harold on time (4)
I believe the answer is:
halt
'stop' is the definition.
(halt can mean to stop)
'harold on time' is the wordplay.
'harold' becomes 'Hal' (diminutive, as in Hal Prince).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'time' becomes 't' (abbreviation).
'hal'+'t'='HALT'
(Other definitions for halt that I've seen before include "Stop or pause" , "Cease" , "Stop; minor station" , "limping" , "Limp; minor station" .)