Great man, but his eccentric aunts are out of money (6)
I believe the answer is:
sultan
'great man but his' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'eccentric aunts are out of money' is the wordplay.
'eccentric' is an anagram indicator.
'are out of' indicates putting letters inside (some letters on the outside of others).
'money' becomes 'l' (L can mean 'pounds').
'aunts' with letters rearranged gives 'sutan'.
'sutan' placed around 'l' is 'SULTAN'.
Can you help me to learn more?
(Other definitions for sultan that I've seen before include "Sovereign of a Muslim state" , "Ottoman sovereign" , "one ruling the roost?" , "One governs" , "Chicken" .)