The long-suffering alto is having to change key in it (7)
I believe the answer is:
stoical
'the long-suffering' is the definition.
(I've seen this before)
'alto is having to change key in it' is the wordplay.
'having to change' is an anagram indicator (I've seen 'has to change' mean this).
'key' becomes 'c' (musical key).
'in it' means one lot of letters goes inside another.
'alto'+'is'='altois'
'altois' with letters rearranged gives 'stoial'.
'stoial' enclosing 'c' is 'STOICAL'.
(Other definitions for stoical that I've seen before include "resigned to suffering" , "Enduring hardship without complaining" , "Long-suffering" , "Indifferent to pain and pleasure" , "patient, > uncomplaining" .)