The expression of a hard-faced expert (7)
I believe the answer is:
grimace
'the expression' is the definition.
(I've seen this before)
'a hard-faced expert' is the wordplay.
'a hard faced' becomes 'grim' (I can't justify this - if you can you should give a lot more credence to this answer).
'expert' becomes 'ace' (ace can informally mean very good).
'grim'+'ace'='GRIMACE'
'of' is the link.
Can you help me to learn more?
(Other definitions for grimace that I've seen before include "Nasty expression" , "Distorted facial expression" , "(Make) a wry face" , "Pained expression" , "displeasure" .)