Idiot returns to sample tart (6)
I believe the answer is:
pastry
'tart' is the definition.
(I've seen this in another clue)
'idiot returns to sample' is the wordplay.
'idiot' becomes 'sap' (sap can mean a foolish or gullible person).
'returns' says the letters should be written in reverse.
'to sample' becomes 'try' (synonyms).
'sap' in reverse letter order is 'pas'.
'pas'+'try'='PASTRY'
(Other definitions for pastry that I've seen before include "See 7 Down" , "Kind of baked food" , "Crust of pies, tarts etc." , "Cooked dough" , "Dough made from flour, fat and water" .)