River the same, not entirely different (6)
I believe the answer is:
thames
'river' is the definition.
(English river)
'the same not entirely different' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'different' indicates an anagram (letters placed in a different order).
'the'+'same'='thesame'
'thesame' with its last letter taken off is 'thesam'.
'thesam' is an anagram of 'THAMES'.
(Other definitions for thames that I've seen before include "River of southern England" , "Longest English river" , "River at Runnymede" , "Capital's river" , "Windsor's river" .)