A key position offering superiority and edge (9)
I believe the answer is:
advantage
'edge' is the definition.
(I've seen this before)
'a key position offering superiority' is the wordplay.
'key' becomes 'd' (I've seen this in other clues).
'position offering superiority' becomes 'vantage' (as in a vantage point).
'a'+'d'+'vantage'='ADVANTAGE'
'and' acts as a link.
(Other definitions for advantage that I've seen before include "Upper hand, benefit" , "Asset" , "Use" , "More favorable position" , "Edge, head start" .)