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 a hidden word.
'retrospective' is a reversal indicator.
'ortni' can be found hidden inside 'fortnightly'.
'ortni' written backwards gives '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" .)