But a well-built vegetarian can be so! (5)
I believe the answer is:
beefy
'so' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'but a well-built vegetarian can be' is the wordplay.
I cannot really see how this works, but
'be' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for beefy that I've seen before include "Muscularly well-developed" , "Solid" , "Strong and muscular" , "Muscular, robust" , "Burly" .)