Sign name for an arachnid (8)
I believe the answer is:
scorpion
'an arachnid' is the definition.
(scorpion is a kind of arachnid)
'sign name' is the wordplay.
I cannot quite understand how this works, but
'sign' could be 'scorpio' (Scorpio is an example) and 'scorpio' is found in the answer.
'name' could be 'n' (common abbreviation e.g. on forms) and 'n' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'for' acts as a link.
(Other definitions for scorpion that I've seen before include "As above (Latin)" , "Desert arachnid" , "Creature with a sting in its tail" , "Eight sign of Zodiac, kind of spider" , "Arachnid with venomous sting" .)