Officer commanding old volunteers left base (5)
I believe the answer is:
octal
'base' is the definition.
(I've seen this before)
'officer commanding old volunteers left' is the wordplay.
'officer commanding' becomes 'oc' (this could be a standard abbreviation I've not previously seen).
'old volunteers' becomes 'TA' (Territorial Army, former name of the Army Reserve).
'left' becomes 'l' (common abbreviation).
'oc'+'ta'+'l'='OCTAL'
Can you help me to learn more?
(Other definitions for octal that I've seen before include "(Of numbers) in base 8" , "Using 8 as base (maths)" , "that must omit a couple of digits" , "based on eights" , "Relating to the number 8" .)