500 teal out in river mouth (5)
I believe the answer is:
delta
'river mouth' is the definition.
(I've seen this before)
'500 teal out in' is the wordplay.
'500' becomes 'd' (Roman numeral for 500).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'in' indicates putting letters inside.
'teal' is an anagram of 'elta'.
'd' put within 'elta' is 'DELTA'.
(Other definitions for delta that I've seen before include "first used by Daedalus" , "Divided river outlet" , "Daedalus started with this" , "Split river mouth system" , "Area of alluvial deposit" .)