Idiom Definition:
  
    
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"to have a leg up"  
    to have an advantage
Synonyms: 
  
advance, advancement, advantage, ahead, begin, burst, dash, early, edge, erupt, gain, go, improve, improvement, jump, lead, leg-up, lunge, rush, spring, stampede, superiority
   
   
  
  
This man, literally, 
has a leg up mounting his horse .... 

see also:
have a head start