Grenade is being tossed — get ready! (2,5)
I believe the answer is:
en garde
'get ready' is the definition.
(phrase used in fencing)
'grenade is being tossed' is the wordplay.
'is being tossed' indicates anagramming the letters.
'grenade' is an anagram of 'EN GARDE'.
(Other definitions for en garde that I've seen before include "Get ready to fence" , "Words of warning" , "Warning cry in fencing" , "Prepare to fence!" , "(In fencing) in a defensive stance" .)