Push hard to break confidence (6)
I believe the answer is:
thrust
'push' is the definition.
(thrusting is a kind of pushing)
'hard to break confidence' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to break' is an insertion indicator.
'confidence' becomes 'trust' (synonyms).
'h' placed into 'trust' is 'THRUST'.
(Other definitions for thrust that I've seen before include "Push forward with force - gist" , "Drive sharply forward" , "Sudden lunge or attack" , "Force producing motion" , "Lunge; propulsive force" .)