One thought unlikely to win, away team against second in group (8)
I believe the answer is:
outsider
'one thought unlikely to win' is the definition.
(eg an outsider is thought unlikely to win a race)
'away team against second in group' is the wordplay.
'away' becomes 'out' (similar in meaning).
'team' becomes 'side' (both can mean a sports team).
'against' says to put letters next to each other.
'second in group' becomes 'r' (2nd letter of 'group').
'out'+'side'+'r'='OUTSIDER'
(Other definitions for outsider that I've seen before include "One not fancied to win" , "Not one of the in-crowd" , "One not in a particular group" , "Competitor rated as unlikely to succeed" , "One unlikely to win" .)