Wrongly declare failure on mountain-top (8)
I believe the answer is:
misspeak
'wrongly declare' is the definition.
I can't tell whether this definition defines the answer.
'failure on mountain-top' is the wordplay.
'failure' becomes 'miss' (miss is a kind of failure).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'mountain top' becomes 'peak' (peak of a mountain).
'miss'+'peak'='MISSPEAK'
Can you help me to learn more?
(Other definitions for misspeak that I've seen before include "say, wrongly" , "Emulate Mrs Clinton" , "say, the wrong way" , "Express oneself badly" .)