Heard bucket is lacking colour (4)
I believe the answer is:
pale
'lacking colour' is the definition.
(I've seen this before)
'heard bucket' is the wordplay.
'heard' shows a homophone (sound like).
'bucket' becomes 'pail' (synonyms).
'pail' sounds like 'PALE'.
'is' acts as a link.
(Other definitions for pale that I've seen before include "Without much colour" , "Stake, lacking colour" , "White" , "Dim - wan" , "Colourless" .)