Reasonable to run away (5)
I believe the answer is:
legit
'run away' is the definition.
(as in 'leg it')
I don't understand the remainder of the clue.
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" .)