Scottish monster is seen swimming! (6)
I believe the answer is:
nessie
'scottish monster' is the definition.
(I've seen this before)
'is seen swimming' is the wordplay.
'swimming' indicates anagramming the letters.
'is'+'seen'='isseen'
'isseen' anagrammed gives 'NESSIE'.
(Other definitions for nessie that I've seen before include "Nickname for a Scottish monster" , "Scots loch monster (colloq.)" , "Supposed loch monster" .)