Change will finally come up after some time (5)
I believe the answer is:
later
'after some time' is the definition.
(I've seen this in another clue)
'change will finally come up' is the wordplay.
'change' becomes 'reta' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'finally' suggests the final letters.
'come up' is a reversal indicator.
The last letter of 'will' is 'l'.
'reta'+'l'='retal'
'retal' backwards is 'LATER'.
Can you help me to learn more?
(Other definitions for later that I've seen before include "at a future date" , "Alert (anag.)" , "after an interval" , "Afterward, subsequent" , "more modern" .)