Eliminates, when one switches off the engine (4,3)
I believe the answer is:
cuts out
'eliminates' is the definition.
(cutting out is a kind of eliminating)
'one switches off the engine' is the wordplay.
I cannot really understand how this works, but
'switches' could be 'cuts' (cutting is a kind of switching) and 'cuts' is present in the answer.
'off' could be 'out' (both can mean wrong or inaccurate) and 'out' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'when' acts as a link.
Can you help me to learn more?
(Another definition for cuts out that I've seen is " extracts from newspaper?".)