Ms Davis has river improved (6)
I believe the answer is:
better
'improved' is the definition.
(similar in meaning)
'ms davis has river' is the wordplay.
I cannot really see how this works, but
'davis' could be 'bette' (I've seen this in another clue) and 'bette' is found in the answer.
'river' could be 'r' (abbreviation for river) and 'r' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for better that I've seen before include "showing improvement" , "More desirable; gambler" , "Surpass; gambler" , "person making use of 43 [TOTE]" , "cap" .)