Gulag has thirty guards filled with terror (6)
I believe the answer is:
aghast
'filled with terror' is the definition.
(I've seen this before)
'gulag has thirty guards' is the wordplay.
'guards' says the answer is hidden in the clue.
'AGHAST' can be found hidden inside 'gulag has thirty'.
(Other definitions for aghast that I've seen before include "Filled with terror" , "Filled with horror or shock" , "Confounded" , "Filled with fear or horror" , "Taken aback with horror" .)