A first: corps replaced by Australian pilot (7)
I believe the answer is:
aviator
'pilot' is the definition.
(I've seen this before)
'a first corps replaced by australian' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is located in the answer.
'by' could be 'at' and 'at' is located in the answer.
The remaining letters 'vor' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for aviator that I've seen before include "Aircraft pilot" , "Navigator of the air" , "Ace, say," , "Flier" , "Woman in a flying machine" .)