That is what one might do with broken-down car (2,3)
I believe the answer is:
to wit
'that is' is the definition.
('to wit' can mean 'namely' or 'that is')
'one might do with broken-down car' is the wordplay.
I cannot really understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'with' could be 'w' (abbreviation) and 'w' is located in the answer.
'car' could be 't' and 't' is present in the answer.
The remaining letters 'ot' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'what' is the link.
Can you help me to learn more?
(Other definitions for to wit that I've seen before include "Namely" , "In other words" , "what to do with a broken-down car!" , "specifically" , "i.e." .)