Jack twice finds deposit (6)
I believe the answer is:
tartar
'deposit' is the definition.
(I've seen this before)
'jack twice' is the wordplay.
'jack' becomes 'tar' (I've seen this before).
'twice' indicates the doubling of some letters.
'tar' doubled is 'TARTAR'.
'finds' is the link.
(Other definitions for tartar that I've seen before include "Formidable person" , "For instance, Genghis Khan" , "Hard, crusty deposit on the teeth" , "plaque" , "A ferocious person" .)