Stories finish with a happy ending, you'll observe (4)
I believe the answer is:
espy
'observe' is the definition.
(I've seen this in another clue)
'stories finish with a happy ending you'll' is the wordplay.
'stories' becomes 'lies' (to tell stories can mean to lie).
'finish' indicates taking half.
'with' says to put letters next to each other.
'a happy ending you'll' becomes 'py' (I can't justify this - if you can you should believe this answer much more).
'lies' halved is 'es'.
'es'+'py'='ESPY'
Can you help me to learn more?
(Other definitions for espy that I've seen before include "Spot at a distance" , "Detect" , "Watch" , "Pick out at a distance" , "Observe, notice" .)