50 per cent of satellites orbited off course in region of Solar System (8,4)
I believe the answer is:
asteroid belt
'region of solar system' is the definition.
'50 per cent of satellites orbited off course' is the wordplay.
'50 per cent of' suggests halving the letters.
'off course' is an anagram indicator (letters go off course from their original places).
'satellites' with half the letters taken is 'satel'.
'satel'+'orbited'='satelorbited'
'satelorbited' with letters rearranged gives 'ASTEROID BELT'.
'in' acts as a link.
(Other definitions for asteroid belt that I've seen before include "Zone where many minor planets orbit" , "Region of interplanetary space - battered soil (anag)" , "Region of space between Mars and Jupiter - besotted liar (anag)" .)