Not bright, extremely dopey, to cross Welsh river! (5)
I believe the answer is:
dusky
'not bright' is the definition.
(dusky can mean dark or dim)
'extremely dopey to cross welsh river' is the wordplay.
'extremely' suggests removing the centre.
'to cross' is an insertion indicator.
'welsh river' becomes 'usk' (I've seen this before).
'dopey' with its middle taken out is 'dy'.
'dy' placed around 'usk' is 'DUSKY'.
(Other definitions for dusky that I've seen before include "Dark-complexioned" , "Swarthy" , "Dark shaded" , "Shadowy, darkish" , "Shadowy or dark-coloured" .)