Stuck in mail, a telegram yet to arrive (4)
I believe the answer is:
late
'yet to arrive' is the definition.
'stuck in mail a telegram' is the wordplay.
'stuck in' indicates a hidden word.
'LATE' can be found hidden inside 'mail a telegram'.
(Other definitions for late that I've seen before include "(Of a person) no longer alive" , "Teal (anag.)" , "Tardy, slow" , "After the usual time" , "Previous" .)