Simulated attack's outside, on board Iron Duke (5)

'simulated' is the definition.
(to fake is to forge or simulate)

'attack's outside on board iron duke' is the wordplay.
'outside' means to remove the middle letters.
'on board' is an insertion indicator (inserted letters are taken on board).
'iron' becomes 'Fe' (Fe is the chemical symbol for iron).
'duke' becomes 'd' (abbreviation).
'attack' with its centre taken out is 'ak'.
'ak' placed into 'fe' is 'fake'.
'fake'+'d'='FAKED'