Great ruler Henry pursues around border (6)
I believe the answer is:
caliph
'border' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'great ruler henry pursues around' is the wordplay.
'great ruler' becomes 'lip' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'pursues' means one lot of letters go next to another.
'around' becomes 'ca' (circa).
'lip'+'h'='liph'
'liph' after 'ca' is 'CALIPH'.
Can you help me to learn more?
(Other definitions for caliph that I've seen before include "Title of rulers of the Islamic world" , "religious leader" , "Harun al-Rashid, say" , "Sultan of Turkey" , "Islamic leader" .)