Swampy ground damages hospital (5)
I believe the answer is:
marsh
'swampy ground' is the definition.
(I've seen this before)
'damages hospital' is the wordplay.
'damages' becomes 'mars' (marring is a kind of damaging).
'hospital' becomes 'h'.
'mars'+'h'='MARSH'
(Other definitions for marsh that I've seen before include "Wetland" , "Wet land; swamp" , "Low-lying wet land with grassy vegetation" , "Quagmire" , "eg Romney" .)