Reputed monster is seen at sea (6)
I believe the answer is:
nessie
'reputed monster' is the definition.
'nessie' can be an answer for 'monster' (Nessie is an example). I am not sure about the 'reputed' bit.
'is seen at sea' is the wordplay.
'at sea' indicates an anagram.
'is'+'seen'='isseen'
'isseen' with letters rearranged gives 'NESSIE'.
Can you help me to learn more?
(Other definitions for nessie that I've seen before include "Supposed loch monster" , "Nickname for a Scottish monster" , "Scots loch monster (colloq.)" .)