Peeved by the description "An expert on weapons" (2,2,4)
I believe the answer is:
up in arms
'peeved' is the definition.
(I've seen this in another clue)
'description an expert on weapons' is the wordplay.
I cannot quite see how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
'weapons' could be 'arms' ('arm' can be a synonym of 'weapon') and 'arms' is located in the answer.
This explanation may well be incorrect...
'by the' is the link.
Can you help me to learn more?
(Other definitions for up in arms that I've seen before include "Weapons expert should be" , "Better prepared for battle" , "Protesting angrily - with weapons aloft?" , "Indignant" , "Rioting" .)