Section of gulag has them overcome with horror (6)
I believe the answer is:
aghast
'overcome with horror' is the definition.
'section of gulag has them' is the wordplay.
'section of' says the answer is hidden in the clue.
'AGHAST' can be found hidden inside 'gulag has them'.
(Other definitions for aghast that I've seen before include "Filled with fear or horror" , "in awe" , "Taken aback with horror" , "Filled with terror" , "Thunderstruck" .)