River mouth, a mess to some extent (6)
I believe the answer is:
thames
'river' is the definition.
(I've seen this before)
'mouth a mess to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'THAMES' can be found hidden inside 'mouth a mess'.
(Other definitions for thames that I've seen before include "Sam the river" , "One going to live abroad" , "London's river (6)" , "'Old Father ___'" , "The Ma's in London" .)