4 across
Drug
no
good
when
engaging
in
battle
(8)
Drug no good when engaging in battle (8)
hints
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hints
The definition is at the beginning of the clue.
How the clue works
Look at Drug at the start, which is your definition, since warfarin is an anticoagulant medicine. For the wordplay, "engaging in battle" gives you WARFARING, and then "no good" tells you to take out the G (the usual abbreviation for good), which leaves you with the drug.