In senior moment initially thought one may be shot by officer (7)
I believe the answer is:
sidearm
'one may be shot by officer' is the definition.
(type of gun)
'in senior moment initially thought' is the wordplay.
'in' indicates putting letters inside.
'senior' becomes 'sr' (similar in meaning).
'initially' says to take the initial letters.
'thought' becomes 'idea' (idea is a kind of thought).
The first letter of 'moment' is 'm'.
'sr'+'m'='srm'
'srm' placed around 'idea' is 'SIDEARM'.
(Other definitions for sidearm that I've seen before include "Piece" , "One may be fired" , "Officer may carry this" , "weapon" .)