Seasonally flooded lowland (5)
I believe the answer is:
swamp
'seasonally flooded lowland' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for swamp that I've seen before include "Engulf" , "Quagmire; inundate" , "Boggy ground" , "Slough" , "Bog, marsh" .)