Old note terminating military commission early (5)
I believe the answer is:
breve
'old note' is the definition.
'breve' can be an answer for 'note' (musical note). I am unsure of the 'old' bit.
'terminating military commission early' is the wordplay.
I cannot really see how this works, but
'military' could be 're' (Royal Engineers) and 're' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for breve that I've seen before include "Rarely used musical note" , "Accent placed over a vowel" , "(In music) long note" , "Note with the time value of eight crotchets" , "Note equivalent to four minims" .)