In all fairness, nothing but hard water (7)
I believe the answer is:
justice
'in all fairness' is the definition.
'justice' can be an answer for 'fairness' (thesaurus). I am unsure of the 'in all' bit.
'nothing but hard water' is the wordplay.
'nothing but' becomes 'just' (I can't justify this - if you can you should believe this answer much more).
'hard water' becomes 'ice'.
'just'+'ice'='JUSTICE'
Can you help me to learn more?
(Other definitions for justice that I've seen before include "Judge, magistrate" , "Courts' function" , "It's dispensed over the bar" , "Fair play" , "Equity" .)