Old Mediterranean leader with a lot of concern about rising ocean (6)
I believe the answer is:
caesar
'old mediterranean leader' is the definition.
'Caesar' can be an answer for 'leader' (Caesar is an example). I am unsure of the 'old mediterranean' bit.
'lot of concern about rising ocean' is the wordplay.
'lot of' means to remove the last letter (a lot of the word but not all of it).
'concern' becomes 'care' (synonyms).
'about' indicates putting letters inside.
'rising' is a reversal indicator (in a down clue, letters go up).
'ocean' becomes 'sea' (synonyms).
'sea' in reverse letter order is 'aes'.
'care' with its final letter taken away is 'car'.
'car' enclosing 'aes' is 'CAESAR'.
'with a' is the link.
Can you help me to learn more?
(Other definitions for caesar that I've seen before include "Julius, stabbed by Brutus" , "Title of the Roman emperors" , "US comedian" , "A scare for Roman emperor" , "leader who acts like a dictator" .)