Amend to "a teal swimming on the river" (5)
I believe the answer is:
alter
'amend' is the definition.
(I've seen this before)
'a teal swimming on the river' is the wordplay.
'swimming' indicates anagramming the letters.
'on' says to put letters next to each other.
'the river' becomes 'r' (abbreviation for river).
'teal' with letters rearranged gives 'alte'.
'alte'+'r'='ALTER'
'to' is the link.
(Other definitions for alter that I've seen before include "Change or adjust" , "Adapt" , "Become different" , "Vary" , "Alert to change" .)