One against appearing in experiment of little importance (7)
I believe the answer is:
trivial
'of little importance' is the definition.
(I've seen this before)
'one against appearing in experiment' is the wordplay.
'one' becomes 'i' (Roman numeral).
'against' becomes 'v' (abbreviation for 'versus').
'appearing in' means one lot of letters goes inside another.
'experiment' becomes 'trial' (trial is a kind of experiment).
'i'+'v'='iv'
'iv' put inside 'trial' is 'TRIVIAL'.
(Other definitions for trivial that I've seen before include "Insignificant" , "Of little substance or significance" , "Frivolous" , "Rinky-dink" , "Rival it, being unimportant" .)