Round bees, terribly fat (5)
I believe the answer is:
'fat' is the definition.
(similar in meaning)
'round bees terribly' is the wordplay.
'round' becomes 'o' (o is a circular letter).
'terribly' indicates anagramming the letters.
'bees' is an anagram of 'bese'.
(Other definitions for obese that I've seen before include "Unusually fat" , "Very substantial" , "in need of slimming down" , "Huge" , "Unhealthily fat and overweight" .)