Available reserve (8)
I believe the answer is:
resource
'available reserve' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for resource that I've seen before include "Stock of something to draw upon" , "Anything that's there to assist when needed" , "Means of support" , "Possibility of help" , "Ingenuity - expedient - supply" .)