Part of system protecting casual worker (4)
I believe the answer is:
temp
'casual worker' is the definition.
(I've seen this before)
'part of system protecting' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'TEMP' is hidden within 'system protecting'.
(Other definitions for temp that I've seen before include "Short-time worker" , "Non-permanent worker" , "N on-permanent worker" , "Non-full time employee" , "Casual worker" .)