Found swapping Sierra and parking - I'm on drive (7)
I believe the answer is:
impetus
'drive' is the definition.
(impetus is a drive to do something)
'found swapping sierra and parking i'm' is the wordplay.
I cannot quite understand how this works, but
'sierra' could be 's' (phonetic alphabet: alpha, bravo, charlie etc.) and 's' is found in the answer.
'parking' could be 'p' (abbreviation on road signs) and 'p' is found in the answer.
'found' could be 'met' (meet can mean to find or encounter) and 'met' is present in the remaining letters.
'i'm' could be 'i' (I've seen this in other clues) and 'i' is located in the answer.
A single letter 'u' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'on' is the link.
Can you help me to learn more?
(Other definitions for impetus that I've seen before include "Driving force, momentum" , "Incentive - time's up (anag)" , "Driving force, forward movement" , "Thrust" , "Moving force" .)