Oncology nerd is confused about one branch of physiology (13)

I believe the answer is:

endocrinology

'branch of physiology' is the definition.

'oncology nerd is confused about one' is the wordplay.
'is confused' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'oncology'+'nerd'='oncologynerd'
'oncologynerd' with letters rearranged gives 'endocrnology'.
'endocrnology' placed around 'i' is 'ENDOCRINOLOGY'.

I've seen this clue in The Times.

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