Fight Club line used in film retrospective (6)
I believe the answer is:
battle
'fight' is the definition.
(I know that fight can be written as battle)
'club line used in film retrospective' is the wordplay.
'club' becomes 'bat' (as in sporting equipment).
'line' becomes 'l' (used when specifying particular lines from a poem).
'used in' is an insertion indicator.
'film' becomes 'ET' (E.T. the Extra-Terrestrial).
'retrospective' says the letters should be written in reverse.
'l' inserted within 'et' is 'elt'.
'elt' back-to-front is 'tle'.
'bat'+'tle'='BATTLE'
(Other definitions for battle that I've seen before include "Difficult conflict or struggle" , "A fight between large forces" , "Military conflict" , "Warfare" , "Major engagement" .)