The phrase "slight favorites" means that someone or something has a slightly higher chance of winning or succeeding compared to others. Full definition
They are slight favourites given the away goals rule but we have a decent chance. (goonerholic.com)
The latest Premier League odds from Betfair places City as slight favourites for the victory at 2/5. (8bit-football.com)
In fact, hosts Wolves, who were crowned League One champions in April, are slight favourites for the match, according to the bookmakers. (caughtoffside.com)