Parasite given heroin by old doctor (10)

I believe the answer is:

horseleech

'parasite' is the definition.
(type of leech)

'heroin by old doctor' is the wordplay.
'heroin' becomes 'horse' (synonyms).
'by' says to put letters next to each other.
'old doctor' becomes 'leech' (I've seen this before).
'horse'+'leech'='HORSELEECH'

'given' is the link.

I've seen this clue in The Times.

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