Simple arms designed for detective (4,6)
I believe the answer is:
miss marple
'detective' is the definition.
'simple arms designed' is the wordplay.
'designed' indicates an anagram (designed with the letters).
'simple'+'arms'='simplearms'
'simplearms' with letters rearranged gives 'MISS MARPLE'.
'for' is the link.
(Other definitions for miss marple that I've seen before include "Agatha Christie detective" , "Agatha Christie's detective" .)