In fairness, it's merely 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'm not sure about the 'in' bit.
'merely hard water' is the wordplay.
'merely' becomes 'just' (synonyms).
'hard water' becomes 'ice' (I've seen this in another clue).
'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 "Judge, magistrate" , "One of man's 22 20 [SEVEN AGES]" , "right outcome" , "Rightness" , "amends" .)