There it's altered as casual garment (3,5)
I believe the answer is:
tee shirt
'casual garment' is the definition.
(I've seen this before)
'there it's altered' is the wordplay.
'altered' indicates an anagram.
'there'+'its'='thereits'
'thereits' with letters rearranged gives 'TEE-SHIRT'.
'as' is the link.
(Other definitions for tee shirt that I've seen before include "Casual garment" , "casual wear" , "top?" , "Simple garment" .)