Devious trap-door hides head of hairy spider, say (9)
I believe the answer is:
arthropod
'spider say' is the definition.
(spider is a kind of arthropod)
'devious trap-door hides head of hairy' is the wordplay.
'devious' indicates an anagram.
'hides' means one lot of letters goes inside another.
'head of' suggests taking the first letters.
The initial letter of 'hairy' is 'h'.
'trap'+'door'='trapdoor'
'trapdoor' with letters rearranged gives 'artropod'.
'artropod' going around 'h' is 'ARTHROPOD'.
(Other definitions for arthropod that I've seen before include "Creature such as insect or crustacean" , "One has external skeleton" , "Invertebrate" , "worker, perhaps" , "spider, for example" .)