Speed of light and then boom and collapse (7)
I believe the answer is:
crumble
'collapse' is the definition.
(I know that collapse can be written as crumble)
'speed of light and then boom' is the wordplay.
I cannot quite see how this works, but
'speed of light' could be 'c' (as in E=mc2) and 'c' is located in the answer.
The remaining letters 'rumble' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'and' acts as a link.
Can you help me to learn more?
(Other definitions for crumble that I've seen before include "Fall to pieces - dessert" , "Break down, fall apart" , "Fall to bits; a pudding" , "Break into small fragments" , "Gradually disintegrate" .)