Military command concerning performance (5,4)
I believe the answer is:
about turn
'military command' is the definition.
(I've seen this before)
'concerning performance' is the wordplay.
'concerning' becomes 'about' (about can mean concerning or relating to).
'performance' becomes 'turn' (turn is a kind of performance).
'about'+'turn'='ABOUT-TURN'
(Other definitions for about turn that I've seen before include "Reversal of direction" , "Act of pivoting 180 degrees" , "Total change of direction" , "Major change of attitude" , "Contrary progression" .)