Ma wanting marsh tits represented in casual clothing (1-6)
I believe the answer is:
t-shirts
'casual clothing' is the definition.
(I've seen this before)
'ma wanting marsh tits represented' is the wordplay.
'wanting' is a deletion indicator (to want something can mean to lack it or need it).
'represented' indicates anagramming the letters.
'marsh'+'tits'='marshtits'
'marshtits' with 'ma' removed is 'rshtits'.
'rshtits' is an anagram of 'T-SHIRTS'.
'in' acts as a link.
(Other definitions for t-shirts that I've seen before include "Simple tops" , "clothes" , "Leisure tops" , "Casual, collarless tops" , "Thirsts (anag)" .)