Chap with current account? He’s lost it (6)
I believe the answer is:
maniac
'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).
'man'+'i'+'ac'='MANIAC'
(Other definitions for maniac that I've seen before include "Crazy person behaving wildly" , "Lunatic or person behaving wildly" , "Very reckless person" , "fruitcake?" , "Nutter" .)