Passage one’s going to read out (5)
I believe the answer is:
aisle
'passage' is the definition.
(aisle is a kind of passage)
'one's going to read out' is the wordplay.
'one' becomes 'i' (Roman numeral).
'going to' becomes 'll' (as in he'll, we'll etc).
'read out' indicates a 'sounds like' (homophone) clue.
'i'+'ll'='ill'
'ill' sounds like 'AISLE'.
(Other definitions for aisle that I've seen before include "way to the altar?" , "Walkway between seats" , "lateral strip" , "Passageway in theatre" , "Passage between seating areas" .)