Returning in Cadillac: a placid South American creature (6)
I believe the answer is:
alpaca
'south american creature' is the definition.
(alpacas are native to South America)
'returning in cadillac a placid' is the wordplay.
'returning' is a reversal indicator.
'in' says the answer is hidden in the clue.
'acapla' can be found hidden inside 'cadillac a placid'.
'acapla' reversed gives 'ALPACA'.
(Other definitions for alpaca that I've seen before include "Woolly animal" , "Andean beast of burden" , "Llama-like creature" , "Silky cloth" , "S American mammal in the llama family" .)