Psycho guy, one with power (6)
I believe the answer is:
maniac
'psycho' is the definition.
(I've seen this before)
'guy one with power' is the wordplay.
'guy' becomes 'man ' (guy can informally mean a man).
'one' becomes 'I' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'power' becomes 'ac' (AC is an example).
'man'+'i'+'ac'='MANIAC'
(Other definitions for maniac that I've seen before include "fruitcake?" , "Mad (keen) type" , "Insane person" , "Nutter" , "Very reckless person" .)