We don't have synonyms for first leg yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are first lien and first law.
(n) :
(sports, soccer) The first of the two matches or “legs” which are played between two teams in a two-legged tie. The winning team is usually determined by aggregate score, the sum of the scores of the two legs.
(n) :
(sports, soccer) The first of the two matches or “legs” which are played between two teams in a two-legged tie. The winning team is usually determined by aggregate score, the sum of the scores of the two legs.