Arranging is tougher for singing brothers (9)
I believe the answer is:
righteous
'singing brothers' is the definition.
(The Righteous Brothers were a singing duo)
'arranging is tougher' is the wordplay.
'arranging' indicates anagramming the letters.
'is'+'tougher'='istougher'
'istougher' anagrammed gives 'RIGHTEOUS'.
'for' is the link.
(Other definitions for righteous that I've seen before include "Moral - virtuous" , "Showing virtue" , "Virtuous, free from sin or guilt" , "Morally justified" , "Good!" .)