Against old soldier from battery unit (4)
I believe the answer is:
volt
'battery unit' is the definition.
'volt' can be an answer for 'unit' (volt is a kind of unit). I am not certain of the 'battery' bit.
'against old soldier' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'soldier' becomes 'lt'.
'v'+'o'+'lt'='VOLT'
'from' acts as a link.
Can you help me to learn more?
(Other definitions for volt that I've seen before include "SI unit of electromotive force" , "Electrical SI unit named after a pioneering Italian physicist, d. 1827" , "A measure of electricity" , "Unit of electric force" , "little force" .)