Part of some terrible measure (5)
I believe the answer is:
meter
'measure' is the definition.
(I know that measure can be written as meter)
'part of some terrible' is the wordplay.
'part of' says the answer is hidden in the clue.
'METER' is hidden within 'some terrible'.
(Other definitions for meter that I've seen before include "Device for recording, say, usage (5)" , "Measuring device, say for electricity usage" , "Measuring device as for electricity usage (5)" , "Device to measure quantity used of fluid, electricity etc" , "Device that measures, say, electricity usage" .)