Attempted new diet? That's about right (5)
I believe the answer is:
tried
'attempted' is the definition.
('try' can be a synonym of 'attempt')
'new diet? that's about right' is the wordplay.
'new' is an anagram indicator.
'that's about' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'diet' anagrammed gives 'tied'.
'tied' enclosing 'r' is 'TRIED'.
(Other definitions for tried that I've seen before include "Attempted" , "heard in court" , "Tested" , "Put to the test" , "tasted" .)