Basis for soup — last of celery with plenty of beef? (6)
I believe the answer is:
stocky
'with plenty of beef?' is the definition.
(stocky can mean sturdy or beefy)
'basis for soup last of celery' is the wordplay.
'basis for' means one lot of letters go next to another.
'soup' becomes 'stock' (stock is a kind of soup).
'last of' says to take the final letters.
The last letter of 'celery' is 'y'.
'stock'+'y' is 'STOCKY'.
(Other definitions for stocky that I've seen before include "Thickset" , "Of sturdy build" , "Small and well built" , "Of short and solid stature" , "Short but well-built" .)