In fairness, it's only hard water (7)
I believe the answer is:
justice
'in fairness' is the definition.
'justice' can be an answer for 'fairness' (I've seen this before). I am not certain of the 'in' bit.
'only hard water' is the wordplay.
'only' becomes 'just' (synonyms).
'hard water' becomes 'ice'.
'just'+'ice'='JUSTICE'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for justice that I've seen before include "One of man's 22 20 [SEVEN AGES]" , "It's dispensed over the bar" , "Rightness" , "Judge, magistrate" , "Fair play" .)