Completely fail to strike stone behind (3,4,6)
I believe the answer is:
hit rock bottom
'completely fail' is the definition.
'hit rock bottom' can be an answer for 'fail' (thesaurus). I'm not sure about the 'completely' bit.
'strike stone behind' is the wordplay.
I cannot really see how this works, but
'strike' could be 'bottom' (bottoming is a kind of striking) and 'bottom' is present in the answer.
'stone' could be 'rock' (synonyms) and 'rock' is found in the answer.
The remaining letters 'hit' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for hit rock bottom that I've seen before include "Be in the worst possible situation" , "Reach one's lowest ebb" .)