Hospital in Paris is most sound (6)
I believe the answer is:
sanest
'most sound' is the definition.
'hospital in paris is' is the wordplay.
'hospital' becomes 'san' (I've seen this in other clues).
'in paris is' becomes 'est' ('is' in French).
'san'+'est'='SANEST'
(Other definitions for sanest that I've seen before include "Most rational and logical" , "Most rational and reasonable" , "Most mentally well" , "Most sensible" , "Assent, being most rational" .)