Swamp primarily surrounding Irish lake (6)
I believe the answer is:
slough
'swamp' is the definition.
(slough is a kind of swamp)
'primarily surrounding irish lake' is the wordplay.
'primarily' indicates taking the first letters.
'irish lake' becomes 'lough' (I've seen this before).
The initial letter of 'surrounding' is 's'.
's'+'lough'='SLOUGH'
(Other definitions for slough that I've seen before include "Swamp; cast off (skin)" , "poet wanted destroyed" , "Swamp or marshy place, maybe of despond" , "shed covering" , "Swamp; Berkshire town" .)