Taxonomist’s flies come down, time after time (7)

I believe the answer is:

diptera

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'taxonomist's flies' is the definition.
I can't judge whether this defines the answer.

'come down time after time' is the wordplay.
'come down' becomes 'dip' (I can't justify this - if you can you should give a lot more credence to this answer).
'time' becomes 't' (abbreviation).
'after' means one lot of letters go next to another.
'time' becomes 'era' (an era is a period of time).
'dip'+'t'+'era'='DIPTERA'

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(Other definitions for diptera that I've seen before include "Insects" , "Flighty little types" , "Flies" .)

I've seen this clue in The Times.

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