Fast food one regularly consumes (4)
I believe the answer is:
diet
'fast food' is the definition.
'diet' can be an answer for 'fast' (dieting is a kind of fasting). I am not certain of the 'food' bit.
'one regularly consumes' is the wordplay.
'one' becomes 'i' (Roman numeral).
'regularly' becomes 'det' (I am not sure about this - if you are sure you should believe this answer much more).
'consumes' is an insertion indicator.
'i' placed within 'det' is 'DIET'.
Can you help me to learn more?
(Other definitions for diet that I've seen before include "prescribed nourishment?" , "Fast - food - convention" , "Prescribed selection of foods" , "Part of regime" , "Health food - Council" .)