Recklessly harms last coastal ecosystem (4,5)
I believe the answer is:
salt marsh
'coastal ecosystem' is the definition.
'recklessly harms last' is the wordplay.
'recklessly' indicates anagramming the letters (I've seen this in other clues).
'harms'+'last'='harmslast'
'harmslast' with letters rearranged gives 'SALT MARSH'.
(Other definitions for salt marsh that I've seen before include "Land sodden with sea water" , "area near the sea" , "Boggy ground" , "regularly flooded area" , "squishy territory?" .)