Significant relation (7)
I believe the answer is:
telling
I believe this clue is a double definition.
'significant' is the first definition.
(I've seen this before)
'relation' is the second definition.
('telling' can be a synonym of 'relation')
(Other definitions for telling that I've seen before include "Striking, having noticeable effect" , "Counting" , "Powerful - effective" , "of significance" , "Powerful; informing" .)