Important, significant (5)
I believe the answer is:
major
This clue is a double definition.
'important' is the first definition.
(major can mean of great importance)
'significant' is the second definition.
(I've seen this in another clue)
(Other definitions for major that I've seen before include "Officer with crown" , "John --, ex-PM" , "Officer - of greater importance" , "Main course" , "Word to describe key" .)