Heard a large, round rock but is much braver (6)
I believe the answer is:
bolder
'braver' is the definition.
('bold' can be similar in meaning to 'brave')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for bolder that I've seen before include "Less shy" , "More impudent or daring" , "Braver or cheekier" .)