Saudi money? Retreat, retreat! (4)
I believe the answer is:
rial
'saudi money?' is the definition.
(currency of Saudi Arabia)
'retreat retreat' is the wordplay.
'retreat' becomes 'lair' (both can mean a private hideout).
'retreat' says the letters should be written in reverse.
'lair' written backwards gives 'RIAL'.
(Other definitions for rial that I've seen before include "Cash" , "Bit of money" , "Middle Eastern money" , "foreign money" , "Ready" .)