Believe knocker didn't start broken (6)
I believe the answer is:
reckon
'believe' is the definition.
(I know that reckon is a more specific form of believe)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for reckon that I've seen before include "Count or calculate" , "Conker (anag.)" , "Calculate or opine" , "Calculate or conclude" , "what people who count will do" .)