Six in test that's unimportant (7)
I believe the answer is:
trivial
'unimportant' is the definition.
(similar in meaning)
'six in test' is the wordplay.
'six' becomes 'VI' (Roman numerals).
'in' indicates putting letters inside.
'test' becomes 'trial' (synonyms).
'vi' put into 'trial' is 'TRIVIAL'.
'that's' acts as a link.
(Other definitions for trivial that I've seen before include "Of little value" , "Rinky-dink" , "Popular board game" , "Meaningless" , "Mundane" .)