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 quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'with' could be 'w' (abbreviation) and 'w' is found within the answer.
'car' could be 't' and 't' is located in the answer.
The remaining letters 'ot' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'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." .)