Using maximum effort (3-3)
I believe the answer is:
all-out
'using maximum effort' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for all-out that I've seen before include "Whole-hearted (effort)" , "Using every effort - everybody exit" , "Nobody home, or using every possible speed" , "Using every resource" , "Using maximum energy - no one at home?" .)