Reasonable to run away (5)
I believe the answer is:
legit
'run away' is the definition.
(as in 'leg it')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for legit that I've seen before include "Briefly in accordance with law or rules" , "Authorised" , "Honest (abbr)" , "acceptable" , "Quite proper" .)