Satellite broadcaster mostly welcoming new input (7)
I believe the answer is:
sputnik
'satellite' is the definition.
(I know that Sputnik is a type of satellite)
'broadcaster mostly welcoming new input' is the wordplay.
'broadcaster' becomes 'sky' (Sky is a television company).
'mostly' means to remove the last letter (most but not all of the word).
'welcoming' means one lot of letters goes inside another (in sense of 'inviting in').
'new' indicates anagramming the letters.
'sky' with its last letter taken away is 'sk'.
'input' with letters rearranged gives 'putni'.
'sk' placed around 'putni' is 'SPUTNIK'.
(Other definitions for sputnik that I've seen before include "Soviet space orbiter" , "High-flier" , "Early Soviet satellite" , "used to encircle the globe" , "space traveller" .)