Unimportant test the four are taking part in (7)
I believe the answer is:
trivial
'unimportant' is the definition.
(similar in meaning)
'four are taking part in' is the wordplay.
'four' becomes 'iv' (roman numeral).
'are taking part' becomes 'trial' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'iv' put into 'trial' is 'TRIVIAL'.
'test the' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for trivial that I've seen before include "Trifling" , "Quite unimportant and paltry" , "Inconsequential, too minor to be bothered about" , "not significant" , "Very easy" .)