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