Spider rain had disturbed, securing cover initially (8)
I believe the answer is:
arachnid
'spider' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for arachnid that I've seen before include "One of species including spiders, scorpions etc" , "tick for one" , "Spider or scorpion, e.g." , "Class of animal including spiders and scorpions" , "Member of the spider family" .)