Her Victor’s back in Siam! (5)
I believe the answer is:
mavis
'siam' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'her victor's back in' is the wordplay.
'her' becomes 'vi'.
'victor's back' becomes 'mas' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'vi' placed within 'mas' is 'MAVIS'.
Can you help me to learn more?
(Other definitions for mavis that I've seen before include "Girl's name - song thrush" , "bird" , "Fem. name, song thrush" , "Girl" .)