Returning, is in a taxi - that's essential (5)
I believe the answer is:
basic
'essential' is the definition.
(similar in meaning)
'returning is in a taxi' is the wordplay.
'returning' says the letters should be written in reverse.
'in' means one lot of letters goes inside another.
'a taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
'is' going within 'cab' is 'cisab'.
'cisab' written backwards gives 'BASIC'.
'that's' is the link.
(Other definitions for basic that I've seen before include "Radical" , "Forming a foundation" , "Without frills" , "Fundamental instruction's" , "Rudimentary - programming language" .)