Old writer to direct drill formation (4,5)

I believe the answer is:

open order

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'drill formation' is the definition.
'open order' can be an answer for 'formation' (open order is a kind of formation). I am unsure of the 'drill' bit.

'old writer to direct' is the wordplay.
'old' becomes 'o' (abbreviation).
'writer' becomes 'pen' (something which writes).
'to direct' becomes 'order' (I've seen this in another clue).
'o'+'pen'+'order'='OPEN ORDER'

Can you help me to learn more?

(Other definitions for open order that I've seen before include "Military formation" , "closed ranks? On the contrary" .)

I've seen this clue in the Sydney Morning Herald.

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