Favourite car in use (6)

I believe the answer is:

minion

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'favourite' is the definition.

'car in use' is the wordplay.
'car' becomes 'mini' (Mini Cooper).
'in use' becomes 'on' (I can't explain this - if you can you should give a lot more credence to this answer).
'mini'+'on'='MINION'

Can you help me to learn more?

(Other definitions for minion that I've seen before include "Servile follower" , "An archaic servant or inferior (6)" , "Servile assistant" , "Fawning servant" , "Servile underling" .)

I've seen this clue in The Guardian.

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