Chap with current account? He’s lost it (6)
'he's lost it' is the definition.
(person who has gone mad)
'chap with current account?' is the wordplay.
'chap' becomes 'man' (I've seen this before).
'with' says to put letters next to each other.
'current' becomes 'i' (symbol for electric current).
'account?' becomes 'ac' (abbreviation for account).
(Other definitions for maniac that I've seen before include "Crazy person behaving wildly", "fruitcake?", "Can aim (anag.)", "Wild, perhaps violent, type", "Insane person".)