Northern son seen changing? Rubbish! (8)
I believe the answer is:
nonsense
'rubbish' is the definition.
(I've seen this in another clue)
'northern son seen changing?' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'changing?' indicates anagramming the letters.
'son'+'seen'='sonseen'
'sonseen' anagrammed gives 'onsense'.
'n'+'onsense'='NONSENSE'
(Other definitions for nonsense that I've seen before include "Quick look" , "'Foolishness, silliness (8)'" , "Gibberish" , "Having no intelligible meaning" , "Boloney, tosh" .)