Monster I sense swimming around? (6)
I believe the answer is:
nessie
'monster' is the definition.
(Nessie is an example)
'i sense swimming around?' is the wordplay.
'swimming' indicates an anagram.
'around?' indicates putting letters inside.
'sense' with letters rearranged gives 'nesse'.
'i' placed inside 'nesse' is 'NESSIE'.
(Other definitions for nessie that I've seen before include "Scots loch monster (colloq.)" , "Supposed loch monster" , "Nickname for a Scottish monster" .)