Arrangement in support of friend largely (6)
I believe the answer is:
format
'arrangement' is the definition.
(I've seen this before)
'in support of friend largely' is the wordplay.
'in support of' becomes 'for' (I've seen this before).
'friend' becomes 'mate' (mate is a kind of friend).
'largely' means to remove the last letter.
'mate' with its final letter removed is 'mat'.
'for'+'mat'='FORMAT'
(Other definitions for format that I've seen before include "Size, style and shape, of books say" , "Prepare (a disk) for data" , "Shape and style as of book" , "Shape and size as of a book, lay-out" , "Way in which something is presented" .)