Break into a jumbled mass on the top of the heap (5)
I believe the answer is:
smash
'break' is the definition.
(smashing is a kind of breaking)
'a jumbled mass on the top of the heap' is the wordplay.
'a jumbled' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'the top of' says to take the initial letters.
The initial letter of 'heap' is 'h'.
'mass' anagrammed gives 'smas'.
'smas'+'h'='SMASH'
'into' acts as a link.
(Other definitions for smash that I've seen before include "Break completely" , "Violently break into pieces" , "Break suddenly into little pieces" , "Hard tennis shot" , "Overhead tennis shot" .)