Certain type of gem Sue uncovered (4-3)
I believe the answer is:
rose-cut
'certain type of gem' is the definition.
(rose-cut describes the form of some gemstones)
'sue uncovered' is the wordplay.
'sue' becomes 'prosecute' (I've seen this before).
'uncovered' says to take the centre.
The middle of 'prosecute' is 'ROSE-CUT'.
(Other definitions for rose-cut that I've seen before include "Hemispherically-faceted style (of diamond)" , "(Of a gem) having tiny triangular facets" , "(Of gem) finished with triangular faces" .)