Acolyte fastidiously keeping up security (6)
I believe the answer is:
safety
'security' is the definition.
(being secure is being safe)
'acolyte fastidiously keeping up' is the wordplay.
'keeping' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ytefas' is hidden in the letters of 'acolyte fastidiously'.
'ytefas' back-to-front is 'SAFETY'.
(Other definitions for safety that I've seen before include "Protection" , "Freedom from risk or danger, or an acrobat's net" , "Security; sanctuary" , "Motto of the cautious!" , "Freedom from danger or harm" .)