Level-headed criminal concealing weapon (8)
I believe the answer is:
balanced
'level-headed' is the definition.
'criminal concealing weapon' is the wordplay.
'criminal' becomes 'bad'.
'concealing' is an insertion indicator.
'weapon' becomes 'lance' (lance is a kind of weapon).
'bad' placed around 'lance' is 'BALANCED'.
(Other definitions for balanced that I've seen before include "Composed - unbiased" , "Having weight equally distributed" , "In a state of equilibrium" , "Auntie is supposedly" , "Stable - poised" .)