Is seen when swimming (or drunk?) (6)
I believe the answer is:
nessie
'drunk?' is the definition.
The definition and answer can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'is seen when swimming' is the wordplay.
'when swimming' indicates anagramming the letters.
'is'+'seen'='isseen'
'isseen' with letters rearranged gives 'NESSIE'.
'or' acts as a link.
(Other definitions for nessie that I've seen before include "Scots loch monster (colloq.)" , "Supposed loch monster" , "Nickname for a Scottish monster" .)