I eat after I order, primarily (6)
I believe the answer is:
iodine
'i' is the definition.
(I is the chemical symbol for iodine)
'eat after i order primarily' is the wordplay.
'eat' becomes 'dine' (to dine is to eat a meal).
'after' means one lot of letters go next to another.
'primarily' indicates taking the first letters.
The first letter of 'order' is 'o'.
'i'+'o'='io'
'dine' after 'io' is 'IODINE'.
(Other definitions for iodine that I've seen before include "Black/violet element" , "Mild antiseptic" , "Element existing in vapour and crystalline form" , "Element with symbol I" , "Black crystalline element" .)