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' indicates anagramming the letters.
'that's about' means one lot of letters goes inside another.
'right' becomes 'r' (common abbreviation).
'diet' with letters rearranged gives 'tied'.
'tied' placed around 'r' is 'TRIED'.
(Other definitions for tried that I've seen before include "Attempted" , "heard in court" , "Tested" , "Put to the test" , "tasted" .)