Pre-eminent, essentially humane prince (4)
I believe the answer is:
main
'humane prince' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'pre-eminent essentially' is the wordplay.
'pre' becomes 'mai' (I am not sure about this - if you are sure you should believe this answer much more).
'essentially' indicates the central letters.
The central letter of 'eminent' is 'n'.
'mai'+'n'='MAIN'
Can you help me to learn more?
(Other definitions for main that I've seen before include "Principal or high seas" , "Cardinal" , "river in Germany" , "location for 9 [ARMADA]" , "Most important - high seas?" .)