Is in taxi turning over key (5)
I believe the answer is:
basic
'key' is the definition.
(I've seen this before)
'is in taxi turning' is the wordplay.
'in' indicates putting letters inside.
'taxi' becomes 'cab' (synonyms).
'turning' says the letters should be written in reverse.
'is' inserted into 'cab' is 'cisab'.
'cisab' back-to-front is 'BASIC'.
'over' is the link.
(Other definitions for basic that I've seen before include "Simple form of language" , "essential" , "Elementary - without extras" , "Unembellished" , "Spartan" .)