Impulsive military assistant’s cracking plan (6)
I believe the answer is:
madcap
'impulsive' is the definition.
(I know that impulsive can be written as madcap)
'military assistant's cracking plan' is the wordplay.
'military assistant's' becomes 'adc' (I can't explain this - if you can you should believe this answer much more).
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'plan' becomes 'map' (plan can mean a map of something).
'adc' going within 'map' is 'MADCAP'.
Can you help me to learn more?
(Other definitions for madcap that I've seen before include "Amusingly eccentric" , "Lively" , "Given to reckless behaviour" , "terror" , "Insane headgear for wildly impulsive person" .)