In this case, Stan's in a nice mess! (8)
I believe the answer is:
instance
'in this case' is the definition.
'instance' can be an answer for 'case' (synonyms). I'm not certain of the remainder of the definition.
'stan's in a nice mess' is the wordplay.
'in' indicates putting letters inside.
'mess' is an anagram indicator.
'nice' with letters rearranged gives 'ince'.
'stan' going inside 'ince' is 'INSTANCE'.
Can you help me to learn more?
(Other definitions for instance that I've seen before include "A case or example of something" , "Particular case; cite" , "Case - isn't acne (anag)" , "Canniest (anag) - occurrence" , "Cite as an example" .)