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