Seasonal gift is a bonus item (6,3)
I believe the answer is:
easter egg
'seasonal gift' is the definition.
(gift given at Easter)
'bonus item' is the wordplay.
'bonus' becomes 'easter' (I can't justify this - if you can you should give a lot more credence to this answer).
'item' becomes 'egg' (I have seen 'Item of food' mean 'egg' so perhaps 'item' could also mean 'egg').
'easter'+'egg'='EASTER-EGG'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for easter egg that I've seen before include "Sweet for Spring festival" , "Edible seasonal symbol" , "Seasonal fare" , "seasonal treat" , "Seasonal confection" .)