Flat area flanked by hills (6)
I believe the answer is:
valley
'flat area flanked by hills' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for valley that I've seen before include "Flat area of land flanked by hills" , "Depression between ranges of hills" , "See 14" , "Hollow between hills" , "Long depression in the surface of the land" .)