Make happy story up with treasure at the end (5)
I believe the answer is:
elate
'make happy' is the definition.
(I've seen this in another clue)
'story up with treasure at the end' is the wordplay.
'story up' becomes 'late' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' says to put letters next to each other.
'at the end' suggests the final letters.
The final letter of 'treasure' is 'e'.
'late' after 'e' is 'ELATE'.
Can you help me to learn more?
(Other definitions for elate that I've seen before include "Make joyful and proud" , "Fill with optimism" , "Bring joy" , "Make delighted or proud" , "Excite" .)