Leading expert - Frenchman - in hospital department (9)
I believe the answer is:
prominent
'leading' is the definition.
(I've seen this in another clue)
'expert frenchman in hospital department' is the wordplay.
'expert' becomes 'pro' (short for professional).
'frenchman' becomes 'm' (abbreviation for Monsieur).
'hospital department' becomes 'ENT' (Ear, Nose and Throat department).
'pro'+'m'+'in'+'ent'='PROMINENT'
(Other definitions for prominent that I've seen before include "Conspicuous or outstanding" , "Standing out" , "Well-known" , "Conspicuous in position or importance" , "Important; protuberant" .)