Open-mouthed initially, words have failed, seeing eclipse (10)
I believe the answer is:
overshadow
'eclipse' is the definition.
(I know that eclipse can be written as overshadow)
'open-mouthed initially words have failed' is the wordplay.
'mouthed' indicates taking the first letters (I've seen 'mouth' mean this (mouth can mean something's entrance)).
'initially' says to put letters next to each other (some letters go before others).
'failed' is an anagram indicator.
The initial letter of 'open' is 'o'.
'words'+'have'='wordshave'
'wordshave' anagrammed gives 'vershadow'.
'o'+'vershadow'='OVERSHADOW'
'seeing' acts as a link.
(Other definitions for overshadow that I've seen before include "Eclipse" , "Dominate" , "Dwarf" , "Cast gloom on" .)