Tell Philistine to banish love that's weak (6)

I believe the answer is:

infirm

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'that's weak' is the definition.
('infirm' can be a synonym of 'weak')

I don't understand how the remainder of the clue works.

(Other definitions for infirm that I've seen before include "Not well" , "Feeble, decrepit" , "unstable" , "stable? No" , "Not in good health" .)

I've seen this clue in The Guardian.

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