Outsiders from Utrecht filling forms for weapons (9)
I believe the answer is:
cutlasses
'weapons' is the definition.
(cutlass is a kind of weapon)
'outsiders from utrecht filling forms' is the wordplay.
'outsiders from' suggests removing the centre (letters on the outside of the word).
'filling' means one lot of letters goes inside another.
'forms' becomes 'classes' ('class' can be a synonym of 'form').
'utrecht' with its middle removed is 'ut'.
'ut' put within 'classes' is 'CUTLASSES'.
'for' is the link.
(Other definitions for cutlasses that I've seen before include "old naval weapons" , "swords" .)