Casual garments (1-6)
I believe the answer is:
t-shirts
'casual garments' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for t-shirts that I've seen before include "Casual garments" , "Thirsts (anag)" , "clothes" , "Casual, collarless tops" , "Close-fitting casual tops" .)