Awful leech attached to tier of organisation (7)

I believe the answer is:

echelon

I'm a little stuck... Click here to teach me more about this clue!

'tier of organisation' is the definition.
'echelon' can be an answer for 'organisation' (echelon is a kind of organization). I'm unsure of the 'tier of' bit.

'awful leech attached' is the wordplay.
'awful' indicates an anagram.
'attached' becomes 'on' (I've seen this in other clues).
'leech' anagrammed gives 'echel'.
'echel'+'on'='ECHELON'

'to' is the link.

Can you help me to learn more?

(Other definitions for echelon that I've seen before include "Tier" , "Rank - status" , "Level or rank of authority, upper or lower" , "Offset-lines formation" , "Echo Len (anag.)" .)

I've seen this clue in The Times.

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