Thoughtless advice to those who are over-worried (8)
I believe the answer is:
careless
'thoughtless' is the definition.
(similar in meaning)
'advice to those who are over-worried' is the wordplay.
I cannot quite see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
'over' could be 're' and 're' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for careless that I've seen before include "Slipshod" , "Marked by lack of attention" , "impetuous" , "Not careful or thorough enough" , "'Heedless, unthinking (8)'" .)