Crazy chap, by one account (6)
I believe the answer is:
maniac
'crazy' is the definition.
(I've seen this before)
'chap by one account' is the wordplay.
I cannot quite see how this works, but
'chap' could be 'man' (chap can mean a man) and 'man' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'account' could be 'ac' (abbreviation for account) and 'ac' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for maniac that I've seen before include "Psycho" , "fruitcake?" , "Insane person" , "Extremely wild person" , "Nutter" .)