Arm of the Mediterranean (6)
I believe the answer is:
aegean
'arm of the mediterranean' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for aegean that I've seen before include "Sea between Greece/Turkey" , "Greek sea" , "Greek-Turkish sea" , "Sea bordering 20 ac" , "Sea, an arm of the Mediterranean" .)