Support for member of cast (5)
I believe the answer is:
sling
This clue is a double definition.
'support for member' is the first definition.
(support for an injured arm)
'cast' is the second definition.
(I know that cast is a is a more specific form of the action of sling)
'of' acts as a link.
(Other definitions for sling that I've seen before include "Bandage support; hurl" , "American drink" , "Primitive weapon" , "Stone-throwing weapon" , "ballistic weapon" .)