Watch key? Quite something (3-6)
I believe the answer is:
eye-opener
'quite something' is the definition.
'watch key?' is the wordplay.
'watch' becomes 'eye' (eye can mean to watch or look at).
'key?' becomes 'opener' (I've seen this before).
'eye'+'opener'='EYE-OPENER'
(Other definitions for eye-opener that I've seen before include "Revelation or enlightenment - with optical reaction?" , "A startling enlightenment!" , "This surprises greatly - makes you look" , "Startling thing" , "Fact that surprises or enlightens in optical way" .)