Support for simple weapon (5)
I believe the answer is:
I believe this is a double definition.
'support' is the first definition.
(sling is a type of support for member (ie an arm))
'simple weapon' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for sling that I've seen before include "attached to a rifle, perhaps" , "Sweetened drink with spirit base" , "Primitive weapon" , "American drink" , "Arm bandage; hurl" .)