Expert in tracery (3)
I believe the answer is:
'expert' is the definition.
(ace can informally mean very good)
'in tracery' is the wordplay.
'in' indicates a hidden word.
'ACE' is hidden in the letters of 'tracery'.
(Other definitions for ace that I've seen before include "Tennis serve that the receiver is unable to reach" , "One-spotted card" , "Hole in one; very good" , "Outstanding" , "reason for point of no return" .)