Singer in rehearsal, tone-deaf (4)
I believe the answer is:
Here is my best explanation:
'singer' is the definition.
(alto is a kind of singer)
'in rehearsal tone-deaf' is the subsidiary indication.
'in' indicates a hidden word.
'alto' can be found hidden inside 'rehearsal tone deaf'.
(Other definitions for alto that I've seen before include "Singer", "Low female singing voice", "Adult male voice", "Highest male voice: lowest female voice", "Lowest female singing voice", "The highest adult male singing-voice", "Second of four parts (in singing)", "Kind of singing voice, male or female", "very high voice" and "Male singer".)