Southern Irish lake and marsh (6)
I believe the answer is:
slough
'marsh' is the definition.
(thesaurus)
'southern irish lake' is the wordplay.
'southern' becomes 's' (abbreviation).
'irish lake' becomes 'lough' (I've seen this before).
's'+'lough'='SLOUGH'
'and' acts as a link.
(Other definitions for slough that I've seen before include "Swamp; Berkshire town" , "Berkshire town and local authority" , "Swamp; shed (dead skin)" , "A bog or marsh, perhaps of despond" , "Swamp or marshy place, maybe of despond" .)