With all possible speed (7)
'with all possible speed' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for hotfoot that I've seen before include "Without the slightest delay" , "acting quickly?" , "With all possible speed" , "flat out" , "Experienced by firewalker" .)
(Other definitions for posthaste that I've seen before include "As quickly as possible" , "at the double" , "With utmost speed -- to the pass (anag)" , "Speedily" , "Hurry the mail with utmost speed" .)