Timely gift set unwrapped since cart's back before horse (6,3)
I believe the answer is:
easter egg
'timely gift' is the definition.
'easter egg' can be an answer for 'gift' (I have seen 'Seasonal gift ' mean 'easter egg' so perhaps 'gift' could also mean 'easter egg'). I'm unsure of the 'timely' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for easter egg that I've seen before include "Segregate into popular post-Lenten sweet" , "Seasonal confection" , "Edible seasonal symbol" , "Programmer's surprise" , "DVD bonus" .)