Rush at unusual rate (4)
I believe the answer is:
tear
'rush' is the definition.
(I know that rush is a is a more specific form of the action of tear)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tear that I've seen before include "Rate (anag.)" , "It's secreted by the lachrymal gland" , "A rip and a sign of sadness" , "Drop secreted by the lacrimal glands" , "Secretion of the lachrymal glands" .)