Remark that the water isn't hard (6)
I believe the answer is:
notice
'remark that the water isn't hard' is the definition.
'notice' can be an answer for 'remark' (thesaurus). I'm unsure of the rest of the definition.
'remark that the water' is the wordplay.
'remark' becomes 'not' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'that the water' becomes 'ice' (ice water is a kind of water).
'not'+'ice'='NOTICE'
Can you help me to learn more?
(Other definitions for notice that I've seen before include "This is usually given before quitting job" , "Warning - observe - review" , "Observe the placard or sign" , "Bulletin" , "Observe, percieve" .)