Sign to informant, the weed (4,5)

I believe the answer is:

crab grass

'weed' is the definition.
(thesaurus)

'sign to informant' is the wordplay.
'sign' becomes 'crab' ().
'to informant' becomes 'grass' (term for an informant or traitor).
'crab'+'grass'='CRAB GRASS'

'the' is the link.

I've seen this clue in The Daily Express.

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