Dirty resort has improved on the surface (11)
I believe the answer is:
bespattered
'dirty' is the definition.
(similar in meaning)
'resort has improved on the surface' is the wordplay.
I cannot quite see how this works, but
'resort' could be 'spa' (spa is a kind of resort) and 'spa' is found within the answer.
'improved' could be 'bettered' ('better' can be a synonym of 'improve') and 'bettered' is present in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for bespattered that I've seen before include "covered in splashes" , "Splashed with liquid" .)