Hierarchical structure (6)
I believe the answer is:
ladder
'hierarchical structure' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ladder that I've seen before include "A run in a stocking" , "Climbing structure" , "Means of ascent" , "Climbing frame; damage tights" , "wherewithal to rescue cat from tree" .)