Trap door collapsed trapping hard crustacean? (9)
I believe the answer is:
arthropod
'crustacean?' is the definition.
(crustacean is a kind of arthropod)
'trap door collapsed trapping hard' is the wordplay.
'collapsed' indicates anagramming the letters.
'trapping' indicates putting letters inside.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'trap'+'door'='trapdoor'
'trapdoor' is an anagram of 'artropod'.
'artropod' going around 'h' is 'ARTHROPOD'.
(Other definitions for arthropod that I've seen before include "Such as a crustacean or spider" , "spider, for example" , "Invertebrate" , "worker, perhaps" , "One has external skeleton" .)