Atlas for one crossing ancient river to find plant (9)
I believe the answer is:
goldenrod
'plant' is the definition.
The answer is an example.
'atlas for one crossing ancient river' is the wordplay.
'atlas for one' becomes 'god' (atlas is a kind of god).
'crossing' means one lot of letters goes inside another.
'ancient' becomes 'olden' (I've seen this before**).
'river' becomes 'r'.
'olden'+'r'='oldenr'
'god' going around 'oldenr' is 'GOLDENROD'.
'to find' is the link.
(Other definitions for goldenrod that I've seen before include "Autumnal plant" , "bloomer" , "Tall-spiked plant" , "Autumnal flower" , "Yellow-spiked garden flower" .)