Top end of market gives support to this new couturier, ultimately (1-5)
I believe the answer is:
t-shirt
'top' is the definition.
(type of top)
'end of market gives support to this new couturier ultimately' is the wordplay.
'end of market' becomes 't' (I can't explain this - if you can you should believe this answer much more).
'gives support to' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'new' is an anagram indicator.
'ultimately' says to take the final letters.
The last letter of 'couturier' is 'r'.
'this' anagrammed gives 'tshi'.
'tshi'+'r'='tshir'
't' put after 'tshir' is 'T-SHIRT'.
Can you help me to learn more?
(Other definitions for t-shirt that I've seen before include "Casual garment like sleeved vest" , "Close-fitting pullover shirt" , "Simple garment" , "Pullover top" , "Casual upper garment" .)