Putting the king under threat but holding back (8)
I believe the answer is:
'holding back' is the definition.
('check' can be a synonym of 'hold back')
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for checking that I've seen before include "Verifying" , "Ecamining for accuracy, quality, or condition" , "Establishing that something is correct" , "Inspecting for accuracy" .)