Unusually sullen, in dumps, lacking heart with lack of colour (8)
I believe the answer is:
dullness
'lack of colour' is the definition.
'dullness' can be an answer for 'lack' (I have seen 'lack of interest' mean 'dullness' so perhaps 'lack' could also mean 'dullness'). I'm not sure about the 'of colour' bit.
'unusually sullen in dumps lacking heart' is the wordplay.
'unusually' indicates an anagram.
'in' is an insertion indicator.
'lacking heart' means to remove the middle letters.
'dumps' with its middle taken out is 'ds'.
'sullen' with letters rearranged gives 'ullnes'.
'ullnes' going inside 'ds' is 'DULLNESS'.
'with' is the link.
Can you help me to learn more?
(Other definitions for dullness that I've seen before include "lack of interest" , "Slowness in understanding, or boring quality" , "State of being boring or not bright" , "Dreary quality" .)