Rapidly obese eating seconds (4)
I believe the answer is:
fast
'rapidly' is the definition.
(I've seen this before)
'obese eating seconds' is the wordplay.
'obese' becomes 'fat' ('fat' can be similar in meaning to 'obese').
'eating' indicates putting letters inside.
'seconds' becomes 's'.
'fat' placed around 's' is 'FAST'.
(Other definitions for fast that I've seen before include "Eat nothing" , "Quick - don't eat" , "'Quickly, rapidly (4)'" , "Abstain from eating" , "At high speed" .)