Most dignified country is situated near the mouth of the Thames (10)

'most dignified' is the definition.
('stately' can be similar in meaning to 'dignified')

'country is situated near the mouth of the thames' is the wordplay.
'country' becomes 'state' (synonyms).
'is situated' becomes 'lies' (I've seen this in another clue).
'near the' means one lot of letters go next to another.
'mouth of' indicates taking the first letters (mouth can mean something's origin).
The first letter of 'thames' is 't'.
'state'+'lies'+'t'='STATELIEST'