Son seen splashed by new flannel (8)
I believe the answer is:
nonsense
'flannel' is the definition.
(thesaurus)
'son seen splashed by new' is the wordplay.
'splashed' indicates an anagram.
'by' says to put letters next to each other.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'son'+'seen'='sonseen'
'sonseen' anagrammed gives 'onsense'.
'onsense' put after 'n' is 'NONSENSE'.
(Other definitions for nonsense that I've seen before include "'Foolishness, silliness (8)'" , "Language without meaning" , "Foolish words or ideas" , "Claptrap, hogwash" , "Words or ideas that don't make sense" .)