Perhaps designer garments last for one season — gets darker clothes (6)
I believe the answer is:
denims
'perhaps designer garments' is the definition.
(eg denim jeans)
'last for one season gets darker clothes' is the wordplay.
'last for' indicates one should take the final letters.
'gets darker' becomes 'dims' (to dim is to get darker).
'clothes' indicates putting letters inside.
The last letters of 'one season' are 'en'.
'en' inserted within 'dims' is 'DENIMS'.
(Other definitions for denims that I've seen before include "Hard-wearing clothes for work or play" , "hard-wearing trousers" , "Clothes made of strong twilled cotton fabric" , "Jeans" .)