Muslim ruler’s last month in hospital (6)
I believe the answer is:
sultan
'muslim ruler's' is the definition.
(I've seen this before)
'last month in hospital' is the wordplay.
'last month' becomes 'ult' (short for 'last month' in correspondence).
'in' means one lot of letters goes inside another.
'hospital' becomes 'san' (I've seen this before).
'ult' put within 'san' is 'SULTAN'.
(Other definitions for sultan that I've seen before include "Ottoman emperor" , "One governs" , "Sovereign of a Muslim state" , "Muslim ruler" , "one ruling the roost?" .)