Damp and low-lying (6)
I believe the answer is:
marshy
'damp and low-lying' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for marshy that I've seen before include "Long narrow mark" , "Wet and low-lying" , "(Of soil) soft and wet" , "treacherous underfoot" , "Boggy; waterlogged" .)