Monster is seen at sea (6)
I believe the answer is:
nessie
'monster' is the definition.
(I know that Nessie is a legendary creature)
'is seen at sea' is the wordplay.
'at sea' is an anagram indicator.
'is'+'seen'='isseen'
'isseen' is an anagram of 'NESSIE'.
(Other definitions for nessie that I've seen before include "Nickname for a Scottish monster" , "Scots loch monster (colloq.)" , "Supposed loch monster" .)