Area inside American bases (7)
I believe the answer is:
diamond
'area' is the definition.
(diamond is a kind of area)
'american bases' is the wordplay.
I cannot really understand how this works, but
'american' could be 'am' (abbreviation in some contexts) and 'am' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'inside' acts as a link.
Can you help me to learn more?
(Other definitions for diamond that I've seen before include "Carbon gem" , "DOG" , "The hard stuff" , "Carbon gemstone" , "Birthstone for April" .)