He has a pair of jacks (6)
I believe the answer is:
tartar
'he' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'a pair of jacks' is the wordplay.
'a pair of jacks' can mean two substitutions for 'jack'.
'jack' becomes 'tar' (both are informal terms for sailor).
'jack' becomes 'tar' (both are informal terms for sailor).
'tar'+'tar'='TARTAR'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for tartar that I've seen before include "Crusty deposits on teeth" , "Crust on teeth or wine deposit" , "Hard, crusty deposit on the teeth" , "Sauce often accompanying fish" , "ancient Mongolian" .)