Rush at an unreasonable rate (4)

I believe the answer is:

tear

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'rush' is the definition.
(tear is a kind of rush)

'an unreasonable rate' is the wordplay.
'rate' anagrams to 'TEAR'.
However, I'm not clear how the anagram is indicated.

'at' is the link.

Can you help me to learn more?

(Other definitions for tear that I've seen before include "A drop of optical moisture" , "A globule of eye moisture" , "Rip apart; drop" , "charge" , "Rip; salty drop" .)

I've seen this clue in the Evening Standard.

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