Plant of the hour? (4)
I believe the answer is:
rush
I believe this is a double definition.
'plant' is the first definition.
(rush is a type of plant)
'the hour?' is the second definition.
(rush hour is a kind of hour)
'of' is the link.
(Other definitions for rush that I've seen before include "Perform hurriedly; marsh plant" , "Burn rubber" , "Speed along - waterside plant" , "Deal with (something) hurriedly" , "Move fast or attack suddenly" .)