I believe the answer is:
'no-holds-barred' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for all-out that I've seen before include "Using full strength or energy - nobody home?" , "Nobody home, or using every possible speed" , "Using every available energy or speed" , "Using every effort - everybody exit" , "Whole-hearted (effort)" .)