Shock, finding first of falsehoods true (6)
I believe the answer is:
fright
'shock' is the definition.
(I've seen this before)
'first of falsehoods true' is the wordplay.
'first of' says to take the initial letters.
'true' becomes 'right' ('right' is associated in meaning with 'true').
The first letter of 'falsehoods' is 'f'.
'f'+'right'='FRIGHT'
'finding' is the link.
(Other definitions for fright that I've seen before include "A scare or shock" , "Panic" , "Terror" , "Grotesque" , "Sudden feeling of fear" .)