Garment a foot short seen in prison (6)
I believe the answer is:
caftan
'garment' is the definition.
(caftan is a kind of garment)
'a foot short seen in prison' is the wordplay.
'foot short' becomes 'ft' (abbreviation for 'foot').
'seen in' indicates putting letters inside.
'prison' becomes 'can' (the can is American slang for prison).
'a'+'ft'='aft'
'aft' inserted inside 'can' is 'CAFTAN'.
(Other definitions for caftan that I've seen before include "Cotton or silk ankle-length garment with full sleeves worn by men in the Levant" , "Middle-Eastern long dress" , "Loose tunic" , "Long belted male tunic" , "Long loose garment" .)