Scotsman's grass in the river? (7)
I believe the answer is:
potomac
'the river?' is the definition.
(I've seen this before)
'scotsman's grass' is the wordplay.
I cannot really understand how this works, but
'grass' could be 'pot' (cannabis) and 'pot' is present in the answer.
'scotsman' could be 'mac' and 'mac' is found within the answer.
A single letter 'o' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'in' is the link.
Can you help me to learn more?
(Other definitions for potomac that I've seen before include "Washington's river" , "Water flowing" , "River of Washington DC" , "Capital outflow" , "to which banks in Washington have access" .)