The comparison is out by a mile! (6)
I believe the answer is:
simile
'the comparison' is the definition.
(simile is a kind of comparison)
'is out by a mile' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'by' means one lot of letters go next to another.
'is' anagrammed gives 'si'.
'si'+'mile'='SIMILE'
(Other definitions for simile that I've seen before include "Figure of speech comparing one thing to another" , "Basic figure of speech, makes comparison" , "The likening of one thing to another" , "Most common figure of speech, one of comparison" , "device, comparatively speaking" .)