Push belief about heaven at first (6)
I believe the answer is:
'push' is the definition.
(thrusting is a kind of pushing)
'belief about heaven at first' is the wordplay.
'belief' becomes 'trust' (trust is a kind of belief).
'about' means one lot of letters goes inside another.
'at first' suggests taking the first letters.
The first letter of 'heaven' is 'h'.
'trust' enclosing 'h' is 'THRUST'.
(Other definitions for thrust that I've seen before include "Lunge forward" , "Push forward as with sword or spear" , "Push forward with force - gist" , "Lunge; propulsive force" , "Push forward, as fencer does sword" .)