Playing a joke is quite unimportant (8)
I believe the answer is:
trifling
This clue could be a double definition.
'playing a joke' is the first definition.
'trifling' can be an answer for 'playing' (thesaurus). I'm not sure about the 'a joke' bit.
'quite unimportant' is the second definition.
'trifling' can be an answer for 'unimportant' (I've seen this before). I'm unsure of the 'quite' bit.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for trifling that I've seen before include "Dallying; petty" , "Of little importance" , "Paltry" , "Fooling around; unimportant" , "Minor" .)