Rock back in breaking jar (7)
I believe the answer is:
granite
'rock' is the definition.
(type of rock)
'back in breaking jar' is the wordplay.
'back' says the letters should be written in reverse.
'breaking' is an insertion indicator.
'jar' becomes 'grate' (make unpleasant sound).
'in' back-to-front is 'ni'.
'ni' inserted within 'grate' is 'GRANITE'.
(Other definitions for granite that I've seen before include "Constituent in Dartmoor" , "Tangier (anag.)" , "Resilience; stone" , "A very hard rock from Tangier" , "Very hard type of rock" .)