One McCartney say in South Africa (6)
I believe the answer is:
stella
'one mccartney' is the definition.
(I know this)
'say in south africa' is the wordplay.
'say' becomes 'tell' (synonyms).
'in' means one lot of letters goes inside another.
'south africa' becomes 'SA' (abbreviation).
'tell' put inside 'sa' is 'STELLA'.
(Other definitions for stella that I've seen before include "lady?" , "Girl's name meaning star" , "Swift alias for Esther" , "____ McCartney, fashion designer" , "Swift favourite" .)