Cast off in southern Irish lake (6)
I believe the answer is:
slough
'cast off' is the definition.
(sloughing is a kind of casting off)
'southern irish lake' is the wordplay.
'southern' becomes 's' (abbreviation).
'irish lake' becomes 'lough' (I've seen this before).
's'+'lough'='SLOUGH'
'in' acts as a link.
(Other definitions for slough that I've seen before include "Swamp; shed (dead skin)" , "Berkshire town - bog - shed" , "Berkshire town and local authority" , "Discard - bog" , "poet wanted destroyed" .)