She doesn't agree with Billy (5)
I believe the answer is:
nanny
'billy' is the definition.
(thesaurus)
'she doesn't agree' is the wordplay.
'she' becomes 'Nan'.
'doesn't agree' becomes 'ny' (I can't explain this - if you can you should give a lot more credence to this answer).
'nan'+'ny'='NANNY'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for nanny that I've seen before include "Carer for children" , "Child minder - goat - grandma" , "Childminder - goat" , "Producer of milk" , "Posh child minder or female goat" .)