Transport route blocked by a plant (7)
I believe the answer is:
caraway
'plant' is the definition.
(caraway is a kind of plant)
'transport route blocked by a' is the wordplay.
'transport' becomes 'car' (car is a method of transport).
'route' becomes 'way' (way is a kind of route).
'blocked by' is an insertion indicator (block can mean to plug a gap in something).
'car'+'way'='carway'
'carway' going around 'a' is 'CARAWAY'.
(Other definitions for caraway that I've seen before include "____ seeds, for cooking" , "Oil taken from it" , "Seed used as flavouring" , "Seeds used for flavouring" , "Aromatic plant" .)