Most of all, to become a singer (4)
I believe the answer is:
alto
'a singer' is the definition.
(alto is a kind of singer)
'most of all to' is the wordplay.
'most of' means to remove the last letter.
'all' with its final letter taken off is 'al'.
'al'+'to'='ALTO'
'become' acts as a link.
(Other definitions for alto that I've seen before include "High male singing voice" , "Kind of singing voice, male or female" , "Low female singing voice" , "Member of choir" , "Key old > male who scores high" .)