Large having consumed seconds: restrict diet (4)
I believe the answer is:
fast
'diet' is the definition.
(fasting is a kind of dieting)
'large having consumed seconds restrict' is the wordplay.
'large' becomes 'fat' (I've seen this in other clues).
'having consumed' means one lot of letters goes inside another.
'seconds' becomes 'ss' (an abbreviation for seconds written twice).
'restrict' means to remove the last letter (I've seen 'restricted' mean this (restricted can mean limited or reduced)).
'ss' with its last letter removed is 's'.
'fat' placed around 's' is 'FAST'.
(Other definitions for fast that I've seen before include "Speedily - secured" , "Starve; quick" , "Dissolute" , "Abstain from food/drink" , "Can't move" .)