After opener stumped, single that is vital to stability (8)
I believe the answer is:
keystone
'is vital to stability' is the definition.
I can't tell whether this definition defines the answer.
'after opener stumped single' is the wordplay.
'after' says to put letters next to each other.
'opener' becomes 'key' (keys open things).
'stumped' becomes 'st' (cricket abbreviation).
'single' becomes 'one' (a single thing - only one).
'key'+'st' is 'keyst'.
'keyst'+'one'='KEYSTONE'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for keystone that I've seen before include "What tops an arch" , "Locking piece for arch" , "Piece at the top of a masonry arch" , "Central part of arch" , "important element" .)