Son seen swimming around Nessie’s head? Rubbish! (8)
I believe the answer is:
nonsense
'rubbish' is the definition.
(I've seen this before)
'son seen swimming around nessie's head?' is the wordplay.
'swimming' indicates anagramming the letters.
'around' indicates putting letters inside.
'head?' indicates taking the first letters (I've seen 'head' mean this).
The first letter of 'nessie' is 'n'.
'son'+'seen'='sonseen'
'sonseen' anagrammed gives 'nonsese'.
'nonsese' enclosing 'n' is 'NONSENSE'.
(Other definitions for nonsense that I've seen before include "silly ideas" , "Baloney" , "rhubarb" , "Piffle and poppycock" , "hot air" .)