Cast off, beginning to sail on Irish lake (6)
I believe the answer is:
slough
'cast off' is the definition.
(sloughing is a kind of casting off)
'beginning to sail on irish lake' is the wordplay.
'beginning to' suggests taking the first letters.
'on' means one lot of letters go next to another.
'irish lake' becomes 'lough' (I've seen this before).
The first letter of 'sail' is 's'.
's'+'lough'='SLOUGH'
(Other definitions for slough that I've seen before include "poet wanted destroyed" , "Swamp; Berkshire town" , "Discard - bog" , "Swamp; shed (dead skin)" , "Berkshire town - bog - shed" .)