Duck's first teal out in river mouth (5)
I believe the answer is:
delta
'river mouth' is the definition.
(I've seen this before)
'duck's first teal out in' is the wordplay.
'duck's first' becomes 'd' (1st letter of 'duck').
'out' indicates an anagram (out can mean wrong or inaccurate).
'in' indicates putting letters inside.
'teal' with letters rearranged gives 'elta'.
'd' going inside 'elta' is 'DELTA'.
(Other definitions for delta that I've seen before include "Wing shape; Greek D" , "Extended area at river mouth" , "Spreading river mouth" , "Triangular river system" , "Letter - where river divides" .)