Raillery is not good in one long period of time (8)
I believe the answer is:
badinage
'raillery' is the definition.
(I know that badinage is a type of raillery)
'not good in one long period of time' is the wordplay.
'not good' becomes 'bad' (bad is the opposite of good).
'one long period of time' becomes 'age' (I can't explain this - if you can you should believe this answer much more).
'bad'+'in'+'age'='BADINAGE'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for badinage that I've seen before include "Playful banter" , "Teasing talk, banter" , "joking" , "Frivolous repartee" , "Josh" .)