The French Scotsman in faulty car (5)
I believe the answer is:
lemon
'faulty car' is the definition.
(I've seen this in another clue)
'the french scotsman' is the wordplay.
'the french' becomes 'le' ('the' in French).
'scotsman' becomes 'mon'.
'le'+'mon'='LEMON'
'in' acts as a link.
(Other definitions for lemon that I've seen before include "Useless object" , "Leaving body" , "Fruit - useless thing" , "Melon is another fruit" , "Dab" .)