Second in importance (6)
I believe the answer is:
moment
This clue is a double definition.
'second' is the first definition.
(I know that second can be written as moment)
'importance' is the second definition.
(I know that moment is a type of significance)
'in' acts as a link.
(Other definitions for moment that I've seen before include "Second, importance" , "Second, flash" , "An exact point of time" , "Instant - importance" , "A very brief portion of time" .)