Four or six in rehearsal? That's not important (7)
I believe the answer is:
trivial
'not important' is the definition.
'four or six in rehearsal?' is the wordplay.
I cannot quite understand how this works, but
'four' could be 'iv' (Roman numerals for four) and 'iv' is found in the answer.
'rehearsal?' could be 'trial' and 'trial' is found within the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for trivial that I've seen before include "Banal" , "Footling" , "Meaningless" , "Inconsequential, too minor to be bothered about" , "Unimportant, minor" .)