Opening passage part of fortnightly retrospective (5)
I believe the answer is:
intro
'opening passage' is the definition.
(I've seen this before)
'part of fortnightly retrospective' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'retrospective' says the letters should be written in reverse.
'ortni' can be found hidden inside 'fortnightly'.
'ortni' in reverse letter order is 'INTRO'.
(Other definitions for intro that I've seen before include "this may get one started" , "Musical opening" , "First bars of a musical composition" , "overture" , "Prelude" .)