Last on is hardly concealing shock (8)
I believe the answer is:
astonish
'shock' is the definition.
(thesaurus)
'last on is hardly concealing' is the wordplay.
'concealing' indicates the answer is hidden within the clue.
'ASTONISH' can be found hidden inside 'last on is hardly'.
(Other definitions for astonish that I've seen before include "Flabbergast" , "Stagger" , "Stun" , "Dumbfound" , "Shock with surprise" .)