Scare king during crusade (6)
I believe the answer is:
fright
'scare' is the definition.
('fright' can be a synonym of 'scare')
'king during crusade' is the wordplay.
'king' becomes 'r' (r is abbreviation for rex).
'during' is an insertion indicator.
'crusade' becomes 'fight'.
'r' going within 'fight' is 'FRIGHT'.
(Other definitions for fright that I've seen before include "Alarming event" , "Shock, sudden alarm" , "Shock, intense fear" , "Grotesque" , "Sudden exposure to fear" .)