Drops fees, every so often when these quantities are prescribed (5)

I believe the answer is:

doses

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'these quantities are prescribed' is the definition.
The definition suggests an adverb but the answer is not.

'drops fees every so often' is the wordplay.
'every so often' means one should take alternating letters.
'drops'+'fees'='dropsfees'
The alternate letters of 'dropsfees' are 'DOSES'.

'when' is the link.

Can you help me to learn more?

(Other definitions for doses that I've seen before include "Prescribed amounts of medicine" , "Medicine measures" , "Administers medicine to (someone)" , "Medicates" , "Gives medicine to" .)

I've seen this clue in The Irish Times.

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