Subtlety? Good attempt, but not right (6)
I believe the answer is:
nicety
'subtlety?' is the definition.
(I know that subtlety can be written as nicety)
'good attempt but not right' is the wordplay.
'good' becomes 'nice' (similar in meaning).
'attempt' becomes 'try' (synonyms).
'but not' is a deletion indicator.
'right' becomes 'r' (abbreviation).
'try' with 'r' taken out is 'ty'.
'nice'+'ty'='NICETY'
(Other definitions for nicety that I've seen before include "Matter of delicate adjustment" , "Degree of precision" , "Minute distinction; subtlety" , "shade?" , "Fastidiousness" .)