Catchphrase in register kept by hospital (6)
I believe the answer is:
slogan
'catchphrase' is the definition.
(eg a political catchphrase)
'register kept by hospital' is the wordplay.
'register' becomes 'log' (a log is a register).
'kept by' is an insertion indicator.
'hospital' becomes 'san' (short for sanatorium).
'log' going into 'san' is 'SLOGAN'.
'in' acts as a link.
(Other definitions for slogan that I've seen before include "Word or phrase adopted as motto" , "short political statement" , "Catchy phrase in advertisement etc" , "'Motto, watchword (6)'" , "Shibboleth" .)