Pure gunk on top of dustbin (4)
I believe the answer is:
good
'pure' is the definition.
(thesaurus)
'gunk on top of dustbin' is the wordplay.
'gunk' becomes 'goo' (synonyms).
'on top of' says to take the initial letters.
The first letter of 'dustbin' is 'd'.
'goo'+'d'='GOOD'
(Other definitions for good that I've seen before include "Pious" , "and 22 Down and 14 Across: Christmas carol" , "Well done! Health-giving" , "To be desired; pleasing" , "Worthy" .)