Arranging is tougher for singing brothers (9)
I believe the answer is:
Here is my best explanation:
'singing brothers' is the definition.
(The Righteous Brothers were a singing duo)
'arranging is tougher' is the wordplay.
'arranging' indicates anagramming the letters.
'istougher' anagrammed gives 'RIGHTEOUS'.
'for' is the link.
(Other definitions for righteous that I've seen before include "Moral - virtuous", "Just, upright", "Virtuous, free from sin or guilt", "Morally justified", "Good!", "Showing virtue", "law-abiding" and "Morally justifiable, or people who behave thus".)