Woman in 'Fun in Acapulco' (4)
I believe the answer is:
nina
'woman' is the definition.
'in fun in acapulco' is the wordplay.
'in' indicates the answer is hidden within the clue.
'NINA' can be found hidden inside 'fun in acapulco'.
(Other definitions for nina that I've seen before include "One of Columbus's ships in 1492; girl's name" , "hidden crossword message" , "- - Simone, 1933-2003, singer" , "Girl" , "- Simone, US singer" .)