Change to a signorina's make-up (12)
I believe the answer is:
organisation
'make-up' is the definition.
(make-up can mean organisation or structure)
'change to a signorina's' is the wordplay.
'change' indicates anagramming the letters.
'to'+'a'+'signorina'='toasignorina'
'toasignorina' with letters rearranged gives 'ORGANISATION'.
(Other definitions for organisation that I've seen before include "concern" , "System" , "Systematic approach to tasks" , "Running" .)