Announce day finally set (5)
I believe the answer is:
telly
'set' is the definition.
(I know that telly is a type of radio receiver)
'announce day finally' is the wordplay.
'announce' becomes 'tell'.
'finally' indicates one should take the final letters.
The last letter of 'day' is 'y'.
'tell'+'y'='TELLY'
(Other definitions for telly that I've seen before include "cop in seventies" , "The goggle-box" , "- - Savalas" , "Set" , "TV (colloq.)" .)