Discover the truth about collapse? Not initially (6)
I believe the answer is:
rumble
'discover the truth about' is the definition.
(I've seen this before)
'collapse? not initially' is the wordplay.
'collapse?' becomes 'crumble' ('crumble' can be a synonym of 'collapse').
'not initially' means to remove the first letter.
'crumble' with its initial letter removed is 'RUMBLE'.
(Other definitions for rumble that I've seen before include "uncover secrecy perhaps" , "Low continuous noise" , "Harlem gangs might" , "Such a strip" , "Make a continuous resonant sound" .)