Frivolous four interrupt rehearsal (7)
I believe the answer is:
trivial
'frivolous' is the definition.
(similar in meaning)
'four interrupt rehearsal' is the wordplay.
'four' becomes 'IV' (roman numeral).
'interrupt' indicates putting letters inside.
'rehearsal' becomes 'trial'.
'iv' put inside 'trial' is 'TRIVIAL'.
(Other definitions for trivial that I've seen before include "Worthless" , "Of little importance" , "Inconsequential, too minor to be bothered about" , "Frivolous" , "Mundane" .)