Obese, gobbling seconds? Don't eat! (4)
I believe the answer is:
fast
'don't eat' is the definition.
(fasting is not eating)
'obese gobbling seconds?' is the wordplay.
'obese' becomes 'fat' ('fat' can be similar in meaning to 'obese').
'gobbling' is an insertion indicator.
'seconds?' becomes 's'.
'fat' enclosing 's' is 'FAST'.
(Other definitions for fast that I've seen before include "Firm < moving rapidly" , "Tight; quick" , "firm to > go without" , "Not eating food" , "Go hungry" .)