Bath's sex appeal girl (5)
I believe the answer is:
sauna
'bath's' is the definition.
(I've seen this before)
'sex appeal girl' is the wordplay.
'sex appeal' becomes 'sa'.
'girl' becomes 'una' (**).
'sa'+'una'='SAUNA'
(Other definitions for sauna that I've seen before include "Sweat room" , "Room for steam bath" , "Very hot cleansing room from Finland" , "Steam-heated room" , "Very hot room for cleansing, of Finnish origin" .)