Watch next to key, a surprising sight (3-6)
I believe the answer is:
eye-opener
'a surprising sight' is the definition.
'watch next to key' is the wordplay.
'watch' becomes 'eye' (I've seen this before).
'next to' says to put letters next to each other.
'key' becomes 'opener' (I've seen this before**).
'eye'+'opener'='EYE-OPENER'
(Other definitions for eye-opener that I've seen before include "Surprising fact - like wake up call" , "Fact that brings great surprise - wakes you up" , "Surprise revelation" , "Fact or circumstance that brings enlightenment and surprise" , "Surprising fact or circumstance of ocular origin" .)