Puts back by one day, which is foolish (6)
I believe the answer is:
stupid
'is foolish' is the definition.
(synonyms)
'puts back by one day' is the wordplay.
'back' is a reversal indicator.
'by' is a charade indicator (letters next to each other).
'one' becomes 'i' (Roman numeral).
'day' becomes 'd' (abbreviation for day).
'puts' reversed gives 'stup'.
'stup'+'i'+'d'='STUPID'
'which' acts as a link.
(Other definitions for stupid that I've seen before include "Daft" , "potty" , "Dim" , "Unintelligent, really thick" , "(Seeming to be) lacking intelligence" .)