Hospital thanks Saint Nicholas (5)
I believe the answer is:
santa
'saint nicholas' is the definition.
(I've seen this before)
'hospital thanks' is the wordplay.
'hospital' becomes 'san' (short for sanatorium).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'san'+'ta'='SANTA'
(Other definitions for santa that I've seen before include "Saint Nicholas" , "Christmas visitor" , "& 22 Father Christmas" , "The present chap" , "bringer of gifts" .)