Quietly left off plaque with date incorrectly cut for watercourse (8)
I believe the answer is:
aqueduct
'watercourse' is the definition.
(thesaurus)
'quietly left off plaque with date incorrectly cut' is the wordplay.
I cannot really see how this works, but
'date' could be 'd' (common abbreviation eg in DOB, date of birth) and 'd' is found within the answer.
an anagram of 'cut' is 'uct' which is located in the answer.
This explanation may well be incorrect...
'for' is the link.
(Other definitions for aqueduct that I've seen before include "Artificial channel carrying water across country" , "Carrier of water over valley" , "Conduit that carries water over a valley" , "Canal bridge" , "8 across carrying water" .)