I'm wrong, boy, by a very long way! (5)
I believe the answer is:
miles
'i'm wrong boy by a very long way' is the definition.
I can't tell whether this defines the answer.
'i'm wrong boy by' is the wordplay.
'wrong' is an anagram indicator.
'boy' becomes 'les' (short for Leslie).
'by' means one lot of letters go next to another.
'im' with letters rearranged gives 'mi'.
'mi'+'les'='MILES'
Can you help me to learn more?
(Other definitions for miles that I've seen before include "a long way" , "Imperial units of length" , "Distance units" , "'But I have promises to keep, and . . . . . to go before I sleep' (Frost)" .)