Northern son seen recycling garbage (8)
I believe the answer is:
nonsense
'garbage' is the definition.
(thesaurus)
'northern son seen recycling' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'recycling' indicates anagramming the letters.
'son'+'seen'='sonseen'
'sonseen' anagrammed gives 'onsense'.
'n'+'onsense'='NONSENSE'
(Other definitions for nonsense that I've seen before include "Hokum, hogwash" , "Language without meaning" , "Farce" , "Boloney, tosh" , "Quick look" .)