Shock when peeper gets key (3-6)
I believe the answer is:
eye-opener
'shock' is the definition.
(thesaurus)
'peeper gets key' is the subsidiary indication.
'peeper' becomes 'eye' ('eye' can be a synonym of 'peeper').
'gets' says to put letters next to each other.
'key' becomes 'opener' (I've seen this before).
'eye'+'opener'='eyeopener'
'when' is the link.
(Other definitions for eye-opener that I've seen before include "Circumstances that brings surprise and enlightenment" , "Fact that surprises or enlightens in optical way" , "Fact or circumstance that brings enlightenment and surprise" , "Something startling" , "This surprises greatly - makes you look" .)