One under surface, a group of arthropods (9)
I believe the answer is:
crustacea
'group of arthropods' is the definition.
(crustaceans are arthropods)
'one under surface a' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'under' says to put letters next to each other.
'surface' becomes 'crust' (crust can mean something's surface).
'ace' put after 'crust' is 'crustace'.
'crustace'+'a'='CRUSTACEA'
(Other definitions for crustacea that I've seen before include "Collection of hardbacks" , "shellfish" , "crabby group?" , "class of seafood" , "Shelled group" .)