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 see 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' and 'out' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'when' is the link.
Can you help me to learn more?
(Another definition for cuts out that I've seen is " extracts from newspaper?".)