Within half a mile? I don't believe it!
I believe the answer is:
lie
'don't believe it' is the definition.
(I've seen this in another clue)
'within half a mile? i' is the wordplay.
'within' is an insertion indicator.
'half a' suggests halving the letters.
'mile' cut in half is 'le'.
'le' enclosing 'i' is 'LIE'.
(Other definitions for lie that I've seen before include "Mistruth" , "buried" , "situation" , "Fib in alienation" , "False statement" .)