What arachnophobe might do when encountering a spider? Make haste! (4,2,2)
I believe the answer is:
step on it
I believe this clue is a double definition.
'what arachnophobe might do when encountering a spider?' is the first definition.
(an arachnophobe might step on a spider out of fear)
'make haste' is the second definition.
(thesaurus)
(Other definitions for step on it that I've seen before include "improve rate of advance" , "Hurry up, put foot down on pedal" , "Hurry! - set point (anag)" , "Go faster - put foot down" , "Get a move on!" .)