Hero at hospital keeping promise (4)
I believe the answer is:
oath
'promise' is the definition.
(thesaurus)
'hero at hospital keeping' is the wordplay.
'keeping' indicates a hidden word.
'OATH' is hidden in the letters of 'hero at hospital'.
(Other definitions for oath that I've seen before include "Swearword, expletive" , "Curse or solemn word" , "What frustrated people say" , "Swearword or solemn word" , "One sworn" .)