Historian entered vaguely at first in press and TV register (11)

I believe the answer is:

medievalist

I'm a little stuck... Click here to teach me more about this clue!

'historian' is the definition.
(historian of the Middle Ages)

'entered vaguely at first in press and tv register' is the wordplay.
I cannot quite understand how this works, but
'first' could be 'ist' (ist resembles 1st) and 'ist' is found within the answer.
The remaining letters 'medieval' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Times.

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