Artist, one who improved steam engine using French water (7)
I believe the answer is:
watteau
'artist one who' is the definition.
'watteau' can be an answer for 'artist' (I've seen this before). I am unsure of the remainder of the definition.
'improved steam engine using french water' is the wordplay.
'improved steam engine' becomes 'watt' (I can't justify this - if you can you should give a lot more credence to this answer).
'using' means one lot of letters go next to another.
'french water' becomes 'eau' ('water' in French).
'watt'+'eau'='WATTEAU'
Can you help me to learn more?
(Other definitions for watteau that I've seen before include "Jean-Antoine -, seventeenth to eighteenth century French rococo painter" , "French artist" , "French 18th century painter" , "canvasser" .)