Rather fat (5)
(Other definitions for round that I've seen before include "stage of competition" , "Description of Tweedledum" , "rung of the ladder" , "Eighteen holes of golf" , "Item of music - single item of ammunition" .)
'rather fat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for portly that I've seen before include "Rather fat" , "with corporation" , "Plump, corpulent" , "Stout and dignified" , "Rotund" .)