It's taken to be a round form of hat (4)
I believe the answer is:
oath
'it's taken to' is the definition.
I can't tell whether this defines the answer.
'a round form of hat' is the wordplay.
'a round' becomes 'o' (round can mean a round object).
'form of' indicates an anagram (the letters need to be in another 'form').
'hat' anagrammed gives 'ath'.
'o'+'ath'='OATH'
'be' is the link.
Can you help me to learn more?
(Other definitions for oath that I've seen before include "What frustrated people say" , "Vow, sworn word" , "A curse - or a pledge" , "A solemn undertaking to tell the truth" , "bad language" .)