Communications satellite rattles frightfully (7)
I believe the answer is:
telstar
'communications satellite' is the definition.
(I've seen this before)
'rattles frightfully' is the wordplay.
'frightfully' indicates anagramming the letters.
'rattles' with letters rearranged gives 'TELSTAR'.
(Other definitions for telstar that I've seen before include "Distant transmitter" , "Early sixties satellite" , "Communication satellite" , "Communications satellite" , "Old satellite" .)