Ann Miles is out to become a referee's assistant (8)
I believe the answer is:
linesman
'a referee's assistant' is the definition.
(I've seen this before)
'ann miles is out' is the wordplay.
'is out' indicates anagramming the letters (I've seen 'was out' mean this).
'ann'+'miles'='annmiles'
'annmiles' is an anagram of 'LINESMAN'.
'to become' is the link.
(Other definitions for linesman that I've seen before include "Official helping referee - telephone repairer" , "park official" , "Male official with a flag" , "Football official" , "sports official" .)