What you may do with old car of Parisian ploughing into coach (5,2)
I believe the answer is:
trade in
'what you may do with old car' is the definition.
(you might trade in an old car)
'of parisian ploughing into coach' is the wordplay.
'of parisian' becomes 'de' ('of' in French).
'ploughing into' indicates putting letters inside.
'coach' becomes 'train' (both can mean to instruct a sportsperson).
'de' placed inside 'train' is 'TRADE-IN'.
(Other definitions for trade in that I've seen before include "Old car" , "As part of deal, exchange" , "exchange for second-hand article" , "second-hand car perhaps" , "Part-exchange" .)