Mystic was out by half a mile (5)
I believe the answer is:
swami
'mystic' is the definition.
(I've seen this before)
'was out by half a mile' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'by' says to put letters next to each other.
'half a' indicates taking half.
'mile' halved is 'mi'.
'was' anagrammed gives 'swa'.
'swa'+'mi'='SWAMI'
(Other definitions for swami that I've seen before include "How to address Hindu dignitary" , "Teacher abroad" , "Religious master" , "Hindu religious instructor" , "Hindu male religious teacher" .)