Soldier on LSD that works to achieve neutrality (7)

I believe the answer is:

antacid

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'soldier on lsd that works to achieve neutrality' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'soldier on lsd' is the wordplay.
'soldier' becomes 'ant' (soldier ant is a type of ant).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'lsd' becomes 'acid' (synonyms).
'ant'+'acid'='ANTACID'

Can you help me to learn more?

(Other definitions for antacid that I've seen before include "Medicine counteracting gastric juices" , "Stomach-soothing medicine" , "Stomach-settling medicine" , "Indigestion relief" , "heartburn remedy" .)

I've seen this clue in The Independent.

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