Favouring first quarter in mathematical arrangement of data (6)
I believe the answer is:
format
'arrangement of data' is the definition.
(as in a file format)
'favouring first quarter in mathematical' is the wordplay.
I cannot quite see how this works, but
'favouring' could be 'for' (to be 'for' something is to be in favour of it) and 'for' is found in the answer.
'first' could be 'a' (first letter of the alphabet) and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for format that I've seen before include "Shape and size as of a book, lay-out" , "Style or arrangement of procedure" , "Broadsheet or tabloid, say" , "Arrangement or lay-out, say of book" , "Arrangement (data)" .)