King who rebuilt the casualty centre (4)
I believe the answer is:
saul
'king' is the definition.
(I have seen 'King of Israel' mean 'saul' so perhaps 'king' could also mean 'saul')
'rebuilt the casualty centre' is the wordplay.
'rebuilt' is an anagram indicator.
'centre' means to look at the middle letters.
The centre of 'casualty' is 'sual'.
'sual' is an anagram of 'SAUL'.
'who' acts as a link.
(Other definitions for saul that I've seen before include "inspiring leader of Scots" , "Bellow was one" , "Old King" , "- - of Tarsus" , "Paul, pre-conversion" .)