A label fixed to weapon immediately (2,1,6)
I believe the answer is:
at a glance
'immediately' is the definition.
(I've seen this before)
'a label fixed to weapon' is the wordplay.
'label' becomes 'tag' (tag is a kind of label).
'fixed to' means one lot of letters go next to another.
'weapon' becomes 'lance' (lance is a kind of weapon).
'a'+'tag'+'lance'='AT A GLANCE'
(Other definitions for at a glance that I've seen before include "immediately" , "With just a look" .)