Show astonishment about right understanding (5)
I believe the answer is:
grasp
'understanding' is the definition.
(I know that understanding can be written as grasp)
'show astonishment about right' is the wordplay.
'show astonishment' becomes 'gasp' (I've seen this in another clue).
'about' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'gasp' placed around 'r' is 'GRASP'.
(Other definitions for grasp that I've seen before include "Get hold of firmly" , "Seize, hold firmly" , "Catch on" , "Take hold of firmly" , "Clutch" .)