The reason many go wrong, I see (5)
I believe the answer is:
logic
'the reason' is the definition.
(logic is using reason)
'many go wrong i see' is the wordplay.
'many' becomes 'l'.
'wrong' indicates anagramming the letters.
'see' becomes 'C' (the word for the letter, according to Chambers).
'go' with letters rearranged gives 'og'.
'l'+'og'+'i'+'c'='LOGIC'
(Other definitions for logic that I've seen before include "Reasoned argument" , "The science of reasoning from cause to effect" , "Rationale ?" , "Reasoned thinking" , "Reasoned judgement" .)