Rush, we're told, is a weed (4)
I believe the answer is:
tare
'weed' is the definition.
(I've seen this in another clue)
'rush we're told' is the wordplay.
'rush' becomes 'tear' (tearing is a kind of rushing).
'we're told' indicates a 'sounds like' (homophone) clue.
'tear' is a homophone of 'TARE'.
'is a' is the link.
(Other definitions for tare that I've seen before include "Vehicle's empty weight" , "Weight of unloaded vehicle" , "Vetch plant" , "Weight of container makes a vetch" , "Rate a vetch" .)