Controls parasites in post offices (7)

I believe the answer is:

polices

'controls' is the definition.
('police' can be a synonym of 'control')

'parasites in post offices' is the wordplay.
'parasites' becomes 'lice' (lice are parasitic insects).
'in' means one lot of letters goes inside another.
'post offices' becomes 'pos' (PO is an abbreviation for post office).
'lice' going inside 'pos' is 'POLICES'.

I've seen this clue in The Guardian.

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