They're left in German cars reversed by engineers (7)
I believe the answer is:
residua
'they're left' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.
'german cars reversed by engineers' is the wordplay.
'german cars' becomes 'audis' (I've seen this in another clue).
'reversed' says the letters should be written in reverse.
'by' says to put letters next to each other.
'engineers' becomes 're' (Royal Engineers).
'audis' reversed gives 'sidua'.
'sidua' put after 're' is 'RESIDUA'.
'in' is the link.
Can you help me to learn more?
(Another definition for residua that I've seen is " We've been left behind".)