Very old act, nine involved (7)
I believe the answer is:
ancient
'very old' is the definition.
(I've seen this before)
'act nine involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'act'+'nine'='actnine'
'actnine' anagrammed gives 'ANCIENT'.
(Other definitions for ancient that I've seen before include "Historic standard" , "Very old, like some civilizations" , "from olden times" , "Very old, like the poetic mariner" , "Old salt in Coleridge's poem" .)