Check some of the finest emeralds (4)
I believe the answer is:
stem
'check' is the definition.
(both can mean to control or stop)
'some of the finest emeralds' is the wordplay.
'some of' indicates a hidden word.
'STEM' is hidden in the letters of 'finest emeralds'.
(Other definitions for stem that I've seen before include "supporter of flag, for example" , "Tube" , "Stanch" , "Ski turn and plant stalk" , "Basic skiing turn" .)