As a fine description, substantial (5)
I believe the answer is:
hefty
'substantial' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for hefty that I've seen before include "Large, heavy" , "Sizeable" , "With weight" , "Large and beefy" , "Hulking" .)