Bitter, extremely credible, informant (6)
I believe the answer is:
source
'informant' is the definition.
(I know that informant can be written as source)
'bitter extremely credible' is the wordplay.
'bitter' becomes 'sour' (I've seen this in another clue).
'extremely' means to remove the middle letters (letters on the extremes of the word are used).
'credible' with its middle taken out is 'ce'.
'sour'+'ce'='SOURCE'
(Other definitions for source that I've seen before include "Origin or starting point" , "Find" , "Beginning to add" , "Where something originates" , "From which something springs" .)