Display area marks Welsh river (9)
I believe the answer is:
flagstaff
'display area' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'marks welsh river' is the wordplay.
'marks' becomes 'flags' (flagging is a kind of marking).
'welsh river' becomes 'taff' (I've seen this before).
'flags'+'taff'='FLAGSTAFF'
Can you help me to learn more?
(Other definitions for flagstaff that I've seen before include "somewhere in Arizona" , "Pole for the standard" , "Standard bearer" , "Flying Pole" , "Standard-bearer" .)