Very keen leader dismissed by king of Israel (4)
I believe the answer is:
avid
'very keen' is the definition.
(I've seen this before)
'leader dismissed by king of israel' is the wordplay.
'leader' suggests taking the first letters.
'by' means one lot of letters go next to another.
'king of israel' becomes 'avi' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letter of 'dismissed' is 'd'.
'd' put after 'avi' is 'AVID'.
Can you help me to learn more?
(Other definitions for avid that I've seen before include "Eager like diva" , "Mad for diva" , "Greedy like diva" , "In operation" , "Showing active interest and enthusiasm" .)