Feature of expert in opposition (4,2,4)
I believe the answer is:
face to face
'in opposition' is the definition.
'feature of expert' is the wordplay.
'feature' becomes 'facet' (facet is a kind of feature**).
'expert' becomes 'ace' (ace is a kind of expert).
'facet'+'of'+'ace'='FACE-TO-FACE'
(Other definitions for face to face that I've seen before include "In presence of or confrontation with another" , "Aspect of best service" , "Opposite" , "being in personal contact" .)