Address a fraction of this? Irrational! (3)
I believe the answer is:
sir
'address a fraction' is the definition.
'sir' can be an answer for 'address' (I have seen 'formal address' mean 'sir' so perhaps 'address' could also mean 'sir'). I am not certain of the 'a fraction' bit.
'of this? irrational' is the wordplay.
'of' says the answer is hidden in the clue.
'SIR' is hidden within 'this irrational'.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Title for knight or teacher" , "Form of address for a knight" , "Baronet's title" , "Term of address for a gentleman" , "Honorary male title" .)