Junior to follow one expected to lose (8)
I believe the answer is:
underdog
'one expected to lose' is the definition.
(I've seen this before)
'junior to follow' is the wordplay.
'junior' becomes 'under' (I've seen this in other clues).
'to follow' becomes 'dog' (I've seen this before).
'under'+'dog'='UNDERDOG'
(Other definitions for underdog that I've seen before include "Such is unlikely to succeed" , "Grounded (anag)" , "Least favoured competitor" , "One thought unlikely to win" , "Less fancied competitor" .)