I eat under the moon (6)
I believe the answer is:
iodine
'i' is the definition.
('iodine' can be a synonym of 'i')
'eat under the moon' is the wordplay.
'eat' becomes 'dine' (dining is a kind of eating).
'under' says to put letters next to each other.
'the moon' becomes 'io' (moon of Jupiter).
'dine' put after 'io' is 'IODINE'.
(Other definitions for iodine that I've seen before include "Solution applied to wounds as an antiseptic" , "Element existing in vapour and crystalline form" , "Disinfectant made from seaweed" , "Element with symbol I" , "Infamous tablets in case of a nuclear emergency" .)