Old-fashioned mum carrying nine spinning tops (6)
I believe the answer is:
maxima
'tops' is the definition.
(rare plural form of maximum)
'old-fashioned mum carrying nine spinning' is the wordplay.
'old-fashioned mum' becomes 'mama' (old-fashioned term for 'mum').
'carrying' means one lot of letters goes inside another.
'nine' becomes 'ix' ('ix' can be a synonym of 'nine').
'spinning' says the letters should be written in reverse.
'ix' written backwards gives 'xi'.
'mama' going around 'xi' is 'MAXIMA'.
(Other definitions for maxima that I've seen before include "Largest amounts" , "Largest numbers" , "greatest possible amounts" , "extremely high values" , "High points" .)