Has to step off as quickly as possible (9)
I believe the answer is:
posthaste
'as quickly as possible' is the definition.
(I've seen this before)
'has to step off' is the wordplay.
'off' is an anagram indicator.
'has'+'to'+'step'='hastostep'
'hastostep' is an anagram of 'POSTHASTE'.
(Other definitions for posthaste that I've seen before include "With great speed, after hurry" , "Yes!" , "immediately" , "As quickly as possible" , "With great speed, like delivery of letters?" .)