Well containing salt in water, say, fine! (10)
I believe the answer is:
absolutely
'fine' is the definition.
(thesaurus)
'well containing salt in water' is the wordplay.
'well' becomes 'ably' (I've seen this in another clue).
'containing' means one lot of letters goes inside another.
'salt in water' becomes 'solute' (I can't justify this - if you can you should give a lot more credence to this answer).
'ably' placed around 'solute' is 'ABSOLUTELY'.
'say' acts as a link.
Can you help me to learn more?
(Other definitions for absolutely that I've seen before include "Totally" , "You bet!" , "Utterly" , "Yes, indeed!" , "Dead" .)