Probes without warning place threats on board (4,6)
I believe the answer is:
spot checks
'probes without warning' is the definition.
(spot checks are made without warning)
'place threats on board' is the wordplay.
'place' becomes 'spot' (synonyms).
'threats on board' becomes 'checks' (on a chessboard - check endangers a king).
'spot'+'checks'='SPOT-CHECKS'
(Other definitions for spot checks that I've seen before include "teenagers might undertake?" , "they occur without warning" , "Random examinations" , "What dermatologists may do" .)