Crossword Genius

Send fruitlessly from here to post in this box (6)

Ross

I believe the answer is:

pillar

'post in this box' is the definition.
'pillar' can be an answer for 'post' (a pillar is a post or column). I'm not certain of the 'in this box' bit.

I don't understand the remainder of the clue.

(Other definitions for pillar that I've seen before include "person providing reliable support" , "Support structure" , "Upright post" , "Supporting column" , "Stylite would live on it" .)

I've seen this clue in The Guardian.
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