Be nosy about four ladies? (5)
I believe the answer is:
privy
'ladies?' is the definition.
(both can mean a toilet)
'be nosy about four' is the wordplay.
'be nosy' becomes 'pry' (to pry is to investigate nosily).
'about' indicates putting letters inside.
'four' becomes 'IV' (Roman numerals).
'pry' going around 'iv' is 'PRIVY'.
(Other definitions for privy that I've seen before include "Outside lavatory" , "bog" , "Sharing in secret knowledge - toilet" , "Participating in the knowledge of something secret" , "Admitted (to secrets)" .)