Change midstream holding new plan (10)
I believe the answer is:
mastermind
'plan' is the definition.
(masterminding is a kind of planning)
'change midstream holding new' is the wordplay.
'change' is an anagram indicator.
'holding' is an insertion indicator.
'new' becomes 'n'.
'midstream' anagrammed gives 'mastermid'.
'mastermid' going around 'n' is 'MASTERMIND'.
(Other definitions for mastermind that I've seen before include "Planning supremo" , "authority" , "Brainy TV quiz" , "TV knowledge contest" , "Ringleader" .)