Reputed monster is seen to be ridiculous (6)
I believe the answer is:
nessie
'reputed monster' is the definition.
'nessie' can be an answer for 'monster' (Nessie is an example). I'm not sure about the 'reputed' bit.
'is seen to be ridiculous' is the wordplay.
'to be ridiculous' is an anagram indicator.
'is'+'seen'='isseen'
'isseen' anagrammed gives '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" .)