Height of failure? Lie! (8)
'lie' is the definition.
(misspeak can mean to tell a lie)
'height of failure?' is the wordplay.
I cannot quite understand how this works, but
'height' could be 'peak' (synonyms) and 'peak' is present in the answer.
'failure?' could be 'miss' (miss is a kind of failure) and 'miss' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for misspeak that I've seen before include "Emulate Mrs Clinton" , "Express oneself badly" , "say, wrongly" , "say, the wrong way" .)
'height of failure?' is the definition.
'miss' can be an answer for 'failure?' (miss is a kind of failure). I am not sure about the 'height of' bit.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for miss that I've seen before include "Lass" , "Fail to perceive" , "teacher's address" , "failure" , "Young woman" .)