Smile about dirt (5)
I believe the answer is:
slime
'dirt' is the definition.
(thesaurus)
'smile about' is the wordplay.
'about' is an anagram indicator.
'smile' with letters rearranged gives 'SLIME'.
(Other definitions for slime that I've seen before include "Moist slippery stuff" , "Sludge, gloop" , "Gunk" , "Unpleasantly moist and slippery substance" , "Goo" .)