Foolish note at the back (4)
I believe the answer is:
daft
'foolish' is the definition.
(I've seen this before)
'note at the back' is the wordplay.
'note' becomes 'd' (musical note).
'at the back' becomes 'aft' (aft means towards or at the end of a ship).
'd'+'aft'='DAFT'
(Other definitions for daft that I've seen before include "'Silly, foolish (4)'" , "Informally silly or foolish" , "Informally crazy as a brush" , "like a brush?" , "Potty" .)