Period said to be this writer's speciality — nothing in that (7)

I believe the answer is:

miocene

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'period' is the definition.
(I have seen 'Geological period ' mean 'miocene' so perhaps 'period' could also mean 'miocene')

I cannot understand how the remainder of the clue works.

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(Other definitions for miocene that I've seen before include "Certain strata" , "distant epoch" , "Epoch when apes appeared; mice eon (anagram)" , "When apes first appear" , "Geological period" .)

I've seen this clue in The Times.

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