Rock -- new one kept in jar (7)
I believe the answer is:
granite
'rock' is the definition.
(granite is a kind of rock)
'new one kept in jar' is the wordplay.
'new' becomes 'n'.
'one' becomes 'i' (Roman numeral).
'kept in' indicates putting letters inside.
'jar' becomes 'grate' (make unpleasant sound).
'n'+'i'='ni'
'ni' put within 'grate' is 'GRANITE'.
(Other definitions for granite that I've seen before include "Hard igneous rock" , "Coarse-grained igneous rock" , "Showing great resolution" , "Hard stone from Tangier" , "Constituent in Dartmoor" .)