I eat after six, or at regular intervals (6)
I believe the answer is:
iodine
'i' is the definition.
(I is the chemical symbol for iodine)
'eat after six or at regular intervals' is the wordplay.
'eat' becomes 'dine' (to dine is to eat a meal).
'after' says to put letters next to each other.
'at regular intervals' means one should take alternating letters.
'six'+'or'='sixor'
The alternate letters of 'sixor' are 'io'.
'dine' after 'io' is 'IODINE'.
(Other definitions for iodine that I've seen before include "Element existing in vapour and crystalline form" , "Element used in medicine, photography and dyes" , "Solution applied to wounds as an antiseptic" , "Disinfectant made from seaweed" , "Infamous tablets in case of a nuclear emergency" .)