Don't believe what was said before but uncertain (5)
I believe the answer is:
doubt
'don't believe what' is the definition.
I can't tell whether this defines the answer.
'said before but uncertain' is the wordplay.
'said before' becomes 'do' (I can't explain this - if you can you should give a lot more credence to this answer).
'uncertain' indicates an anagram.
'but' with letters rearranged gives 'ubt'.
'do'+'ubt'='DOUBT'
'was' is the link.
Can you help me to learn more?
(Other definitions for doubt that I've seen before include "Feeling of uncertainty about something" , "Have reservations or be uncertain about" , "Uncertainty about the truth" , "Hesitate to believe" , "Question the truth or accuracy of" .)