Scare boxer, finally, during bout (6)
I believe the answer is:
fright
'scare' is the definition.
('fright' can be a synonym of 'scare')
'boxer finally during bout' is the wordplay.
'finally' suggests the final letters.
'during' means one lot of letters goes inside another.
'bout' becomes 'fight' ('fight' can be a synonym of 'bout').
The final letter of 'boxer' is 'r'.
'r' put inside 'fight' is 'FRIGHT'.
(Other definitions for fright that I've seen before include "Panic" , "Sudden exposure to fear" , "Grotesque" , "Sudden feeling of fear" , "Start" .)