Initially, Fawkes in anger causes disarray (5)
I believe the answer is:
rifle
'disarray' is the definition.
(I've seen this before)
'initially fawkes in anger' is the wordplay.
'initially' says to take the initial letters.
'in' is an insertion indicator.
'anger' becomes 'rile' (rile can mean to annoy or anger).
The initial letter of 'fawkes' is 'f'.
'f' put within 'rile' is 'RIFLE'.
'causes' is the link.
(Other definitions for rifle that I've seen before include "Search and rob - with long gun?" , "Winchester" , "Go through in search" , "Gun; sack" , "Long-barrelled weapon, normal army equipment" .)