Clothes engineers wear to shock (7)
I believe the answer is:
Here is my best explanation:
'clothes' is the definition.
('apparel' can be a synonym of 'clothes')
'engineers wear to shock' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'wear' is an insertion indicator (to wear something is to be covered up by it).
'to shock' becomes 'appal' ('appal' can be a synonym of 'shock'**).
're' going inside 'appal' is 'APPAREL'.
(Other definitions for apparel that I've seen before include "whatever people have on", "Gear", "Dress", "Attire", "attire", "costume", "Clothes - garments", "Formal word for clothing", "Garments" and "formal dress?".)