Box containing Irish water with staff’s large cold meal (10,5)

I believe the answer is:

ploughmans lunch

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'box containing irish water' is the definition.
I don't know anything about this answer so I can't judge whether this works.

'with staff's large cold meal' is the wordplay.
'with' indicates putting letters inside.
'staff' becomes 'man' (manning is a kind of staffing).
'large cold' becomes 'ploughs' (I can't justify this - if you can you should believe this answer much more).
'meal' becomes 'lunch' (lunch is a kind of meal).
'man' placed within 'ploughs' is 'ploughmans'.
'ploughmans'+'lunch'='PLOUGHMANS LUNCH'

Can you help me to learn more?

(Other definitions for ploughmans lunch that I've seen before include "bite to eat" , "meal" , "Snack" .)

I've seen this clue in The Hindu.

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