District hospital defended by old man (5)
I believe the answer is:
shire
'district' is the definition.
(thesaurus)
'hospital defended by old man' is the wordplay.
'hospital' becomes 'H' (abbreviation for hospital).
'defended by' means one lot of letters goes inside another.
'old man' becomes 'sire' (I can't explain this - if you can you should give a lot more credence to this answer).
'h' placed into 'sire' is 'SHIRE'.
Can you help me to learn more?
(Other definitions for shire that I've seen before include "The -----: Tolkien's hobbit-land" , "Powerful draught horse" , "Berks" , "Heavy horse" , "English county" .)