Decorator’s coat is long past being mended (5,5)
I believe the answer is:
gloss paint
'decorator's coat' is the definition.
'gloss paint' can be an answer for 'coat' (I have seen 'Shiny coat' mean 'gloss paint' so perhaps 'coat' could also mean 'gloss paint'). I'm not sure about the 'decorator's' bit.
'is long past being mended' is the wordplay.
'being mended' indicates an anagram.
'is'+'long'+'past'='islongpast'
'islongpast' is an anagram of 'GLOSS PAINT'.
Can you help me to learn more?
(Other definitions for gloss paint that I've seen before include "Shiny covering" , "Shiny coat" .)