The rarest eagle? Of course it is! (4,2,3)
I believe the answer is:
hole in one
'the rarest eagle?' is the definition.
The definition suggests a singular noun which matches the answer.
'of course it is' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is present in the answer.
'course' could be 'ne' and 'ne' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for hole in one that I've seen before include "Singular achievement by golfer - literally" , "The best possible shot by a golfer" , "Golfing feat" , "Singular score in golf" .)