Lasting a few years, lawsuit has nine involved (9)
I believe the answer is:
triennial
'lasting a few years' is the definition.
(I've seen this before)
'lawsuit has nine involved' is the wordplay.
'lawsuit' becomes 'trial' (I can't justify this - if you can you should believe this answer much more).
'has' is an insertion indicator.
'involved' indicates anagramming the letters (involved can mean confusing or complex).
'nine' is an anagram of 'ienn'.
'trial' placed around 'ienn' is 'TRIENNIAL'.
Can you help me to learn more?
(Other definitions for triennial that I've seen before include "Recurring every three years" , "lasting a few years" , "Occurring every three years" , "Regularly occurring" .)