It's thick-skinned, colourless and unwanted (5,8)
I believe the answer is:
white elephant
'it's thick-skinned' is the definition.
I can't tell whether this defines the answer.
'colourless and unwanted' is the wordplay.
'colourless' becomes 'white' (similar in meaning).
'and' means one lot of letters go next to another.
'unwanted' becomes 'elephant' (I can't explain this - if you can you should believe this answer much more).
'white'+'elephant'='WHITE ELEPHANT'
Can you help me to learn more?
(Other definitions for white elephant that I've seen before include "liability" , "Troublesome or useless possession" , "Useless, unwanted possession" , "Possession that proves expensive to keep and difficult to get rid of" , "waste of space" .)