Difficulty for retiring opera singer to sack alto (7)
I believe the answer is:
problem
'difficulty' is the definition.
(I know that problem is a type of difficulty)
'retiring opera singer to sack alto' is the wordplay.
I cannot quite understand how this works, but
'sack' could be 'rob' (sack can mean to rob or plunder) and 'rob' is found within the answer.
This explanation may well be incorrect...
'for' acts as a link.
(Other definitions for problem that I've seen before include "A matter hard to solve" , "Something that needs solving" , "One hard to understand, solve" , "Difficulty, poser" , "Challenging question" .)