Just after beginning of false alarm (6)
I believe the answer is:
fright
'alarm' is the definition.
(I've seen this before)
'just after beginning of false' is the wordplay.
'just' becomes 'right' ('right' can be similar in meaning to 'just').
'after' says to put letters next to each other.
'beginning of' indicates taking the first letters.
The first letter of 'false' is 'f'.
'right' after 'f' is 'FRIGHT'.
(Other definitions for fright that I've seen before include "Horror" , "Sudden exposure to fear" , "Panic" , "Shock, intense fear" , "Shock, sudden alarm" .)