Round - round round the top (6)
I believe the answer is:
rotund
'round' is the definition.
(I know that round can be written as rotund)
'round round the top' is the wordplay.
'round' means one lot of letters goes inside another.
'top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letter of 'the' is 't'.
'round' placed around 't' is 'ROTUND'.
(Other definitions for rotund that I've seen before include "'Corpulent, round (6)'" , "Bulging" , "Tubby" , "Large and plump" , "Rather obese" .)