Margaret, having secured victory, is making high-pitched cry (6)
I believe the answer is:
mewing
'making high-pitched cry' is the definition.
(as in a cat mewing)
'margaret having secured victory' is the wordplay.
'margaret' becomes 'meg' (short for Margaret).
'having secured' indicates putting letters inside.
'victory' becomes 'win' (a win is a victory).
'meg' going around 'win' is 'MEWING'.
'is' acts as a link.
(Other definitions for mewing that I've seen before include "Making cat sound" , "Making a kitten's noise" , "Catcalls" , "Bird's calling" .)