Hard covering shock up? Nightmare (8)
I believe the answer is:
nutshell
'hard covering' is the definition.
'nutshell' can be an answer for 'covering' (nutshell is a kind of covering). I'm unsure of the 'hard' bit.
'shock up? nightmare' is the wordplay.
'shock' becomes 'stun' (both can mean to surprise).
'up?' shows that the letters should be reversed in order (in down clue: letters go upwards).
'nightmare' becomes 'hell'.
'stun' in reverse letter order is 'nuts'.
'nuts'+'hell'='NUTSHELL'
Can you help me to learn more?
(Other definitions for nutshell that I've seen before include "covering for seed" , "Hard carapace around small fruits" , "Casing for kernel" , "Hard fruit casing" , "The essential core" .)