Large having eaten seconds, restrict diet (4)
I believe the answer is:
fast
'diet' is the definition.
(fasting is a kind of dieting)
'large having eaten seconds restrict' is the wordplay.
'large' becomes 'fat' (I've seen this in other clues).
'having eaten' is an insertion indicator.
'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 final letter removed is 's'.
'fat' going around 's' is 'FAST'.
(Other definitions for fast that I've seen before include "Eat nothing" , "Renounce food; quick" , "requirement before surgery" , "Rapidly -- dissipated" , "Don't scoff" .)