Charter takes time for chemical substance (7)
I believe the answer is:
reagent
'for chemical substance' is the definition.
(I've seen this in another clue)
'charter takes time' is the wordplay.
'charter' becomes 'rent' (charter can mean to rent or hire).
'takes' means one lot of letters goes inside another.
'time' becomes 'age' (an age is a period of time).
'rent' going around 'age' is 'REAGENT'.
(Other definitions for reagent that I've seen before include "What causes change" , "Chemically-active substance" , "Active chemical" , "Get near (anag) - substance used in chemistry" , "Substance used in chemical analysis" .)