Brat in layette regularly in shock (4,6)
I believe the answer is:
holy terror
'brat' is the definition.
(synonyms)
'layette regularly in shock' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'in' is an insertion indicator.
'shock' becomes 'horror' (I have seen 'Intense shock ' mean 'horror' so perhaps 'shock' could also mean 'horror').
The alternating letters of 'layette' are 'lyte'.
'lyte' inserted within 'horror' is 'HOLY TERROR'.
'in' acts as a link.
(Other definitions for holy terror that I've seen before include "One who puts the fear of God into one?" , "Troublesome child" , "Very tiresome child" .)