Duke at the bottom of quaint British river (5)
I believe the answer is:
tweed
'british river' is the definition.
'tweed' can be an answer for 'river' (river in Scotland). I'm not certain of the 'british' bit.
'duke at the bottom of quaint' is the wordplay.
'duke' becomes 'd' (abbreviation).
'at the bottom of' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'quaint' becomes 'twee' (both can mean sentimental or old-fashioned).
'd' put after 'twee' is 'TWEED'.
Can you help me to learn more?
(Other definitions for tweed that I've seen before include "flows across a border" , "Flower on border" , "Thick woolen fabric" , "Twilled cloth" , "Rough woollen cloth" .)