A base with terrain off Mediterranean (6)
I believe the answer is:
demean
'a base with terrain off mediterranean' is the definition.
I can't tell whether this defines the answer.
'a base with terrain off' is the wordplay.
'a base' becomes 'mean' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'terrain off' becomes 'de' (I can't explain this - if you can you should believe this answer much more).
'mean' after 'de' is 'DEMEAN'.
Can you help me to learn more?
(Other definitions for demean that I've seen before include "Lower in dignity or reputation" , "Lower in dignity or respect" , "disgrace" , "Lower in reputation, shame" , "Lower in reputation, humiliate" .)