How careless to overshoot again! (6)
I believe the answer is:
remiss
'how careless' is the definition.
'remiss' can be an answer for 'careless' (I've seen this before). I am not certain of the 'how' bit.
'overshoot again' is the wordplay.
I cannot quite see how this works, but
'overshoot' could be 'miss' (I've seen this in another clue) and 'miss' is found within the answer.
'again' could be 're' (prefix meaning 'again') and 're' is found within the answer.
No letters remain.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for remiss that I've seen before include "Negligent, lax" , "Neglectful" , "Not taking care" , "At fault" , "Careless and inattentive" .)