Arrangement that's a first (15)

'arrangement' is the definition.
(alphabetisation is a kind of arrangement)

'a first' is the wordplay.
I cannot quite see how this works, but
'a' could be 'alpha' (the Greek letter A) and 'alpha' is located in the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

'that's' acts as a link.

Can you help me to learn more?