Reputed monster is seen swimming (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 certain of the 'reputed' bit.
'is seen swimming' is the wordplay.
'swimming' indicates an anagram (letters swim around into a new order).
'is'+'seen'='isseen'
'isseen' is an anagram of 'NESSIE'.
Can you help me to learn more?
(Other definitions for nessie that I've seen before include "Scots loch monster (colloq.)" , "Nickname for a Scottish monster" , "Supposed loch monster" .)