Moment – parasitic arachnid (4)
I believe the answer is:
tick
I believe this clue is a double definition.
'moment' is the first definition.
(both can mean a very short amount of time)
'parasitic arachnid' is the second definition.
(I've seen this before)
(Other definitions for tick that I've seen before include "Blood-sucking mite or parasitic insect" , "which is irritating!" , "Instant - credit" , "Insect; noise of watch" , "Parasitic insect or clicking sound" .)