It is involved with a plan that is flexible (6)
I believe the answer is:
pliant
'that is flexible' is the definition.
(synonyms)
'it is involved with a plan' is the wordplay.
'is involved with' indicates an anagram (involved can mean confusing or complex).
'it' anagrammed with 'plan' makes 'PLIANT'.
(Other definitions for pliant that I've seen before include "easily modified" , "Adaptable" , "Tractable" , "Flexible, easy to bend" , "docile" .)