Rush in three directions at once! (4)
I believe the answer is:
reed
'rush' is the definition.
(plants found by water)
'in three directions at once' is the wordplay.
'three directions' can mean three substitutions for 'direction'.
'in' is an insertion indicator.
'direction' becomes 'r' (r is abbreviation for right).
'direction' becomes 'e' (E is an example).
'direction' becomes 'e' (E is an example).
'at once' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'r'+'e'='re'
're' going around 'e' is 'ree'.
'ree'+'d'='REED'
Can you help me to learn more?
(Other definitions for reed that I've seen before include "Bilk had one" , "Clarinet part; plant" , "Grass of marshes" , "It grows in water or marshy land" , "Rush" .)