Four make an appearance in a trial of no importance (7)
I believe the answer is:
trivial
'four make an appearance' is the definition.
I can't judge whether this defines the answer.
'in a trial of no importance' is the wordplay.
'in' means one lot of letters goes inside another.
'of no importance' becomes 'vi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trial' enclosing 'vi' is 'TRIVIAL'.
Can you help me to learn more?
(Other definitions for trivial that I've seen before include "Insignificant" , "Frivolous" , "Mundane" , "lacking weight" , "Petty, piffling" .)