Plant poles in Scottish river (5)
I believe the answer is:
tansy
'plant' is the definition.
(tansy is a kind of plant)
'poles in scottish river' is the wordplay.
'poles' becomes 'NS' (North and South poles).
'in' means one lot of letters goes inside another.
'scottish river' becomes 'tay' (the River Tay in Scotland).
'ns' inserted into 'tay' is 'TANSY'.
(Other definitions for tansy that I've seen before include "plant for medicinal use" , "Plant with yellow flowers" , "Aromatic wild plant" , "Flowering plant" , "Herb used as seasoning" .)