Thus, for half an hour, is bitter (4)
I believe the answer is:
sour
'is bitter' is the definition.
(I've seen this in another clue)
'thus for half an hour' is the wordplay.
'thus' becomes 'so' (synonyms).
'for' says to put letters next to each other.
'half an' means to take half the letters.
'hour' cut in half is 'ur'.
'so'+'ur'='SOUR'
(Other definitions for sour that I've seen before include "Turned - morose" , "Sharp- or tart-tasting" , "Having an acid taste or smell" , "Acrid or sullen" , "Acid or caustic" .)