Rush at different rate (4)
I believe the answer is:
tear
'rush' is the definition.
(tear is a kind of rush)
'different rate' is the wordplay.
'different' indicates anagramming the letters.
'rate' with letters rearranged gives 'TEAR'.
'at' is the link.
(Other definitions for tear that I've seen before include "charge" , "Droplet from the eye" , "Optical sign of sorrow" , "Rate (anag.)" , "Product of the lachrymal gland" .)