Small fragment inside causes terrific rumbling (5)
I believe the answer is:
crumb
'small fragment' is the definition.
'crumb' can be an answer for 'fragment' (crumbing is a kind of fragmenting). I'm unsure of the 'small' bit.
'inside causes terrific rumbling' is the wordplay.
'inside' indicates the answer is hidden within the clue.
'CRUMB' is hidden within 'causes terrific rumbling'.
Can you help me to learn more?
(Other definitions for crumb that I've seen before include "A shred of bread" , "Fragment of cake, bread" , "of the farmhouse?" , "offering a little comfort" , "Small particle of bread" .)