Sergeant-major carries one close (7)
I believe the answer is:
sidearm
'sergeant-major carries one close' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'sergeant-major carries one close' is the wordplay.
'sergeant major' becomes 'SM' (abbreviation for sergeant major).
'carries' is an insertion indicator (inserted letters carried inside).
'one' becomes 'i' (Roman numeral).
'close' becomes 'dear' (similar in meaning).
'i'+'dear'='idear'
'sm' placed around 'idear' is 'SIDEARM'.
Can you help me to learn more?
(Other definitions for sidearm that I've seen before include "Piece" , "Officer may carry this" , "weapon" , "One may be fired" .)