Smart outfit? Yes, but smeared with sand (6,4)
I believe the answer is:
sunday best
'smart outfit?' is the definition.
(smart clothes traditionally worn on a Sunday)
'yes but smeared with sand' is the wordplay.
'smeared with' is an anagram indicator.
'yes'+'but'='yesbut'
'yesbut' with its letters mixed up with 'sand' can make 'SUNDAY BEST'.
(Other definitions for sunday best that I've seen before include "Smart garments" , "Smartest clothes" , "(A person's) finest clothes" , "clothes for the day after" , "Clothes kept for special occasions" .)