Empty-headed suggestion not to be so careful (8)
I believe the answer is:
mindless
'empty-headed suggestion not to be so careful' is the definition.
I can't judge whether this defines the answer.
'not to be so careful' is the wordplay.
'not' becomes 'less' (I've seen this in other clues).
'to' means one lot of letters go next to another (I've seen this in other clues).
'be so careful' becomes 'mind' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'less' put after 'mind' is 'MINDLESS'.
Can you help me to learn more?
(Other definitions for mindless that I've seen before include "Silly" , "Acting with no concern for the consequences" , "Acting unthinkingly" , "without reason" , "(Of activity) repetitive and simple" .)