Observing ability to accept brigadier's last command (4,5)
I believe the answer is:
eyes right
'command' is the definition.
(I've seen this before)
'observing ability to accept brigadier's last' is the wordplay.
'observing ability' becomes 'eyesight' (ability to see or observe).
'to accept' indicates putting letters inside.
'last' indicates one should take the final letters.
The final letter of 'brigadier' is 'r'.
'eyesight' going around 'r' is 'EYES RIGHT'.
(Other definitions for eyes right that I've seen before include "Order" , "Look over there" , "Military command" .)