Yankee in retreat used up foreign currency (5)
I believe the answer is:
riyal
'foreign currency' is the definition.
(currency of Saudi Arabia)
'yankee in retreat used up' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' means one lot of letters goes inside another.
'retreat' becomes 'lair' (I've seen this before).
'used up' says the letters should be written backwards (in a down clue, letters go up).
'y' inserted within 'lair' is 'layir'.
'layir' reversed gives 'RIYAL'.
(Other definitions for riyal that I've seen before include "Saudi currency" , "Monetary unit of Qatar, Saudi Arabia and Yemen" , "money for Saudis" , "foreign cash" .)