Most unusual small carriage — it's for madame (8)
I believe the answer is:
scarcest
'most unusual' is the definition.
(I've seen this before)
'small carriage it's for madame' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'carriage' becomes 'car' (I've seen this in other clues).
'it's for madame' becomes 'c'est' ('c'est' is 'it is' in French - eg for 'madame').
's'+'car'+'cest'='SCARCEST'
(Other definitions for scarcest that I've seen before include "In the shortest supply" , "Most unusual" , "Least plentiful or most rare" .)