Ordering a cab is essential (5)
I believe the answer is:
basic
'essential' is the definition.
(similar in meaning)
'ordering a cab is' is the wordplay.
'ordering' is an anagram indicator.
'cab'+'is'='cabis'
'cabis' anagrammed gives 'BASIC'.
(Other definitions for basic that I've seen before include "Fundamental instruction's" , "Fundamental; alkaline" , "Fundamental, forming a starting point" , "Fundamental, forming a starting-point" , "Elementary - without extras" .)