I moon and eat (6)
I believe the answer is:
iodine
'i' is the definition.
(I is the chemical symbol for iodine)
'moon and eat' is the wordplay.
'moon' becomes 'Io' (moon of Jupiter).
'and' means one lot of letters go next to another.
'eat' becomes 'dine' (to dine is to eat a meal).
'io'+'dine'='IODINE'
(Other definitions for iodine that I've seen before include "Solution of a halogen in ethyl alcohol used on wounds as an antiseptic" , "Infamous tablets in case of a nuclear emergency" , "Element forming black crystals or violet vapour" , "Mild antiseptic" , "Halogen element, atomic no 53" .)