Fury after first detecting a peril (6)
I believe the answer is:
danger
'a peril' is the definition.
(I know that peril can be written as danger)
'fury after first detecting' is the wordplay.
'fury' becomes 'anger' (fury is intense anger).
'after' says to put letters next to each other.
'first detecting' becomes 'd' (1st letter of 'detecting').
'anger' put after 'd' is 'DANGER'.
(Other definitions for danger that I've seen before include "Menace" , "Peril, hazard" , "Peril in garde" , "Peril, jeopardy" , "'Peril, risk (6)'" .)