Allowed to set up brilliant communications satellite (7)
I believe the answer is:
telstar
'communications satellite' is the definition.
(I've seen this before)
'allowed to set up brilliant' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'set up' is a reversal indicator (in a down clue, letters go up).
'brilliant' becomes 'star' (I've seen this in other clues).
'let' back-to-front is 'tel'.
'tel'+'star'='TELSTAR'
(Other definitions for telstar that I've seen before include "Early sixties satellite" , "Communications satellite" , "Distant transmitter" , "Old satellite" , "Communication satellite" .)