Significant pressure attached to ruling (8)
I believe the answer is:
pregnant
'significant' is the definition.
(I know that significant can be written as pregnant)
'pressure attached to ruling' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'attached to' is a charade indicator (letters next to each other).
'ruling' becomes 'regnant' (as in a queen regnant, a queen who rules by birth).
'p'+'regnant'='PREGNANT'
(Other definitions for pregnant that I've seen before include "Expecting a baby" , "With child, enceinte" , "Carrying offspring in the womb" , "loaded" , "full of implications" .)