只有…才合理；只有…才正确
it is the right way to treat someone and what they deserve
I think it's only fair to tell you that we have had over 300 applications for this job. 我认为应该告诉你一声，已经有300多人应聘这份工作了。
It's only fair that they should share the profit from the songs they wrote.
It's only fair that the wealthy should contribute more in tax.
It was only fair that I give my landlord plenty of notice that I was planning to leave.
See also: fair adjective (RIGHT)