Garment was falling short, one to cast out (9)
I believe the answer is:
waistcoat
'garment' is the definition.
(waistcoat is a kind of garment)
'was falling short one to cast out' is the wordplay.
'falling short' means to remove the last letter.
'one' becomes 'i' (Roman numeral**).
'out' indicates an anagram (out can mean wrong or inaccurate).
'was' with its last letter taken away is 'wa'.
'to'+'cast'='tocast'
'tocast' is an anagram of 'stcoat'.
'wa'+'i'+'stcoat'='WAISTCOAT'
(Other definitions for waistcoat that I've seen before include "Something worn" , "American's vest" , "Sleeveless garment" , "Clothing" , "A "vest" in the US" .)