To order cab is essential (5)
I believe the answer is:
basic
'essential' is the definition.
('basic' can be similar in meaning to 'essential')
'to order cab is' is the wordplay.
'to order' indicates an anagram.
'cab'+'is'='cabis'
'cabis' anagrammed gives 'BASIC'.
(Other definitions for basic that I've seen before include "Fundamental or rudimentary" , "Fundamental, forming a starting point" , "At the simplest level" , "Simple - computer language" , "No-frills" .)