His other Malbec keeps at constant temperature (10)
I believe the answer is:
isothermal
'at constant temperature' is the definition.
(I've seen this before)
'his other malbec keeps' is the wordplay.
'keeps' says the answer is hidden in the clue.
'ISOTHERMAL' is hidden within 'his other malbec'.
(Other definitions for isothermal that I've seen before include "without change in temperature?" , "at constant temperature" .)