Accountant heading southeast, to write beforehand for container of stationery supplies (6,4)
I believe the answer is:
pencil case
'container of stationery supplies' is the definition.
'pencil case' can be an answer for 'container' (pencil case is a kind of container). I'm unsure of the 'of stationery supplies' bit.
'accountant heading southeast to write beforehand' is the wordplay.
'accountant' becomes 'CA' (abbreviation for chartered accountant).
'heading' says to put letters next to each other (some letters go in front of or head others).
'southeast' becomes 'SE' (synonyms).
'to write' becomes 'pencil' (I can't explain this - if you can you should believe this answer much more).
'beforehand' is a charade indicator (letters next to each other).
'ca' next to 'se' is 'case'.
'case' put after 'pencil' is 'PENCIL-CASE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for pencil case that I've seen before include "Crayons may have been put in this" , "Clean spice (anag.)" , "drawers here?" .)