To order a cab is essential (5)
I believe the answer is:
basic
'essential' is the definition.
(similar in meaning)
'to order a cab is' is the wordplay.
'to order' indicates an anagram.
'cab'+'is'='cabis'
'cabis' with letters rearranged gives 'BASIC'.
(Other definitions for basic that I've seen before include "Fundamental, forming a starting-point" , "Key" , "Simple form of language" , "Plain" , "At the lowest level" .)