Voice mails too regularly (4)
I believe the answer is:
alto
'voice' is the definition.
(alto is a kind of voice)
'mails too regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'mails'+'too'='mailstoo'
The alternate letters of 'mailstoo' are 'ALTO'.
(Other definitions for alto that I've seen before include "Highest male voice: lowest female voice" , "Voice part" , "Low female singing voice" , "One who sings" , "The highest adult male singing-voice" .)