Fail to secure weapon? Not I - it can go off without reason (5,5)
I believe the answer is:
false alarm
'it can go off without reason' is the definition.
(unnecessary warning)
'fail to secure weapon? not i' is the wordplay.
I cannot quite see how this works, but
'secure' could be 'seal' (I've seen this in another clue) and 'seal' is located in the answer.
'weapon?' could be 'arm' (synonyms) and 'arm' is found in the answer.
'i' could be 'a' and 'a' is found in the answer.
This explanation may well be incorrect...
(Other definitions for false alarm that I've seen before include "unfounded scare?" , "Warning proving unnecessary" , "Panic over" .)