Seasonal dupe has thirty days to get dessert (5,4)
I believe the answer is:
april fool
'seasonal dupe' is the definition.
'April fool' can be an answer for 'dupe' (April fool is an example). I'm not sure about the 'seasonal' bit.
'thirty days to get dessert' is the wordplay.
'thirty days' becomes 'april' (I've seen this before).
'to get' is a charade indicator (letters next to each other).
'dessert' becomes 'fool' (type of dessert).
'april'+'fool'='APRIL FOOL'
'has' is the link.
Can you help me to learn more?
(Other definitions for april fool that I've seen before include "Subject of a practical joke" , "Victim of hoax in springtime" , "Gowk" , "12 [POISSON DAVRIL]" , "Seasonal victim" .)