First half of gift on day one is separate (5)
I believe the answer is:
apart
'one is separate' is the definition.
(similar in meaning)
'first half of gift on day' is the wordplay.
'first' becomes 'a'.
'half of gift on day' becomes 'part' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'part'='APART'
Can you help me to learn more?
(Other definitions for apart that I've seen before include "Unconnected" , "cut off from others" , "Separated or at a distance" , "In pieces; separate" , "Separate in a trap" .)