Reversed slump in American petrol (3)
I believe the answer is:
gas
'american petrol' is the definition.
(I've seen this before)
'reversed slump' is the wordplay.
'reversed' shows that the letters should be reversed in order.
'slump' becomes 'sag' (I've seen this before).
'sag' reversed gives 'GAS'.
'in' is the link.
(Other definitions for gas that I've seen before include "Eg, hydrogen" , "Talk idly; fuel" , "Hydrogen or methane, say" , "natural resource" , "Hydrogen, for example" .)