Small Irish lake or swamp (6)
I believe the answer is:
slough
'swamp' is the definition.
(slough is a kind of swamp)
'small irish lake' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'irish lake' becomes 'lough' (I've seen this before).
's'+'lough'='SLOUGH'
'or' acts as a link.
(Other definitions for slough that I've seen before include "Swamp; shed (dead skin)" , "A bog or marsh, perhaps of despond" , "where friendly bombs were wanted" , "poet wanted destroyed" , "Swamp or marshy place, maybe of despond" .)